Cross-Reference to Related Application

This a continuation-in-part of copending U. S. patent application. Serial No. 07/262,699, filed October 26, 1988, entitled TIME OF TIP-OVER INDICATOR which is now ϋ. S. Patent No. _, , , said application being incorporated herein by reference.

Field of the Invention

The present invention relates to methods and apparatus for time keeping and indicating and, more particularly, to methods and apparatus for an event clock for recording the time of occurrence of an event and for such a device which automatically adjusts for daylight saving time.

Background of the Invention

There are times when it is desirable to detect the times of occurrence of certain actions. Commercial trash haulers sometimes receive complaints from customers that their refuse containers have not been emptied according to their agreed schedules. The trash hauling companies usually require their drivers to keep records of the service calls they make for various business purposes, including documentation that the required calls to customers on a route have been made. The driver may keep such records by

manually filling in an activity log, punching pertinent information into a computerized logging device, or the like. Unforeseeable events may prevent a driver from servicing a customer at a given time, such as traffic tie ups, mechanical breakdown of equipment, inclement weather, or the intervention of holidays. Other, less innocent, factors may also interfere with the process. For example, a customer may have had his container emptied on schedule, only to be refilled by someone other than the rightful customer and without the customer's knowledge. In other cases, a customer may try to obtain more frequent service than contracted for by refilling his timely emptied container and then claiming that it was not emptied on schedule. Finally, an unconcientious driver may log-in service calls not actually made, out of laziness, dishonesty, or some other motive. In order to avoid impugning the honesty of a customer or a driver without objective documentation of service rendered or proof of cheating, the trash hauling company often has no choice but to send a truck on a special trip to empty the customer's container at a non-scheduled time. Such special trips increase operating costs which must be born by other customers or the company. However, the effort required to investigate and prove that someone is cheating in the transaction also has its costs. What is needed is a mechanism for objectively indicating when a particular refuse container was emptied which does not rely on the honesty of a customer or a driver. Such a mechanism must be simple and economical, rugged enough to survive extremes of its operating environment, and tamperproof.

There are other situations in which it is desirable to record the times of occurrence of certain actions, particularly in the servicing of equipment or facilities installed in the field. Examples of services in which time and/or date of service is useful include utility meter reading, the collection of mail from mail deposit boxes and overnight delivery packages from deposit boxes for same, the servicing of portable latrines, and the like. Similarly, a security making rounds in a plant to check the integrity of doors, for example, must often keep records that such doors were checked at a given time on a given date. One problem is that employee kept written records do not provide hard evidence that the employee was ever present at the field location at the time and date recorded. Thus, merely written records can often be disputed, without corroborating evidence. In order to back up such written records and to facilitate recording the information recorded, some service vehicles are provided with service call computers which record the mileage driven, the time stops were made, and may include means for entering other information, such as addresses, customer identification numbers, and the like. Such service call computers are not entirely foolproof. Circumstances, such as mechanical breakdowns, traffic jams, or the like, can occur which might interrupt the ability of the service call computer to accurately record the service worker's progress on a service route. In each of the types of service described above, the facility or site to be serviced is located in the field. What is needed is a means located on the service site which

can be used to record the time and date of each service call. Such a device should include a clock/calendar, preferably with a display, and a switch which is operated by the service worker when the service has been rendered. The device will then provide an indication to the customer, and to the worker's supervisor, of when the last service call was made. If possible, the device should be actuated automatically when access is made to the facility being serviced and should be vitually tamperproof. On the other hand, some means needs to be provided for compensating for daylight saving time. Otherwise, the device will be incorrect by an hour during the time daylight saving time is in effect.

Summary of the Invention

The present invention provides an event clock to record the time of occurrence of an event. Preferably, the event clock incorporates means for correcting for daylight saving time prior to displaying numerals representing the time of occurrence of an event. In one embodiment of the event clock, a time of tip- over indicator is attached to a refuse container to indicate the time and date when the container was last tilted for emptying. The indicator unit has an event switch in the form of a mercury switch which is so positioned that a switch state transition occurs when the refuse container is tilted for emptying. The mercury switch is connected between a battery and a digital clock having a numeric display connected thereto. The change of switch state of

the mercury switch is detected by the digital clock which causes the display to be updated in a selected manner to indicate the time of occurrence of the tilting of the refuse container. In the first tip-over embodiment of the invention, the clock display is updated to the time of day when the tilting occurred. This display is maintained until a subsequent tilting occurs. In a second tip-over embodiment, the clock is reset and restarted in response to the tilting of the refuse container. The clock always restarts to a given time and date, such as 12:00 midnight on January 1. On the display, the numerals are concurrently updated such that the amount of time since the last tilting of the refuse container can be discerned. In each embodiment, month and date numerals are preferably alternated with hour and minute numerals such that no external switch is required to determine the date that dumping of the container ocurred. The time of tip-over indicater unit is housed in a sealed enclosure to prevent tampering with the indicated time and date. A removable front cover is provided to protect the display device from impact damage. The clock circuitry includes a crystal controlled time base generator for accuracy. The components of the circuitry, including the battery and liquid crystal display device, are components which will operate within desired tolerances in the extremes of temperature, humidity, and mechanical shock to which the unit might be subjected. The case of the unit is irremovably attached to a refuse container, as by rivets, to prevent alteration of the indicated time and date.

The event clock of the present invention is also adaptable to detect events by means other than with tilt switches. In a third embodiment of the event clock, the event switch is in the form of a magnetic reed switch which changes states when a magnet is brought in close proximity thereto. One foreseen application for the third embodiment of the event clock employing the magnetic reed switch is to record the time of service of portable enclosed toilets or latrines. In this installation, a clock/display module would preferably be mounted to extend through a locked access panel or be visible through a pane of glass or other transparent material mounted in the access panel. The magnetic reed event switch is mounted inside of the access panel such that it can only be actuated when the access panel is opened to service the portable toilet. Each of the first and third embodiments described above may include a second switch which is actuated to cause the device to display a sequence of previously stored event times, to provide a recent history of service calls. The clock module includes a microprocessor with read/write memory or ram which stores a selected number of event times for this purpose. Each event time is displayed in sequence for a selected length of time, and the latest event time remains displayed when the sequence has completed. Each time a new event time is recorded, the new event time overwrites the oldest event time. In order to insure that times of events are recorded, or at least displayed, accurately, the first and third embodiments of the event clock incorporate a daylight saving time (DST) adjustment mechanism. In an exemplary DST

adjustment mechanism, dates are kept within the clock/calendar device in the form of serial dates numbered from a designated starting date. A table is maintained in memory, preferably read-only memory, of the serial dates for the start and end of daylight saving time for a given succession of years. Whenever it is desirable to display an event time, which includes the date, the recorded serial date is checked by to determine if it falls within a range of dates in which daylight saving time was in effect. If so, an adjustment is made to cause the displayed time advance one hour. Measures are taken to ensure that if the adjusted time causes the displayed time to advance into the next calendar date, the displayed date is also incremented.

Objects of the Invention

The principal objects of the present invention are: to provide a system for detecting and indicating the time and date of the occurrence of an event; to provide such a system which generally operates a time of day clock and which updates a clock display in a particular manner when the switch is operated; to provide such a system which stores the instantaneous time and date that an event switch connected to the clock changes state, and displays the instantaneous time and date; to provide such a system for detecting and indicating a time of occurrence of a tilting operation by the use of a tilt switch; to provide such a system, particularly, for objectively detecting and indicating the time of occurrence of the emptying of a refuse container which is emptied by tilting; to provide

such a system including a mercury tilt switch connecting a battery to a digital clock which maintains the display of numerals representing the time of day of the latest occurrence of a change of switch state of the mercury switch; to provide a first embodiment of such a system wherein the digital clock is implemented as a microprocessor executing a program to function as a digital clock; to provide such a system which is capable of operating with adequate precision in an outdoor environment during all seasons and which is not vulnerable to mechanical shocks; to provide such a system which is virtually tamperproof; to provide a second embodiment of such a system including a mercury switch connecting a battery to a digital clock which, upon being tilted, causes resetting and restarting of the clock to indicate the amount of time which has elapsed since the tilting occurred; to provide a third embodiment of the system in which the event switch is a magnetic reed switch which is caused to change state when a magnet is brought in close proximity to the magnetic switch; to provide such a third embodiment of the system which is mounted on a portable toilet structure to indicate the time of servicing the structure; to provide such a first or third embodiment of the system which includes a memory for storing a history of a plurality of event times which can be cycled through by the operation of a memory display switch; to provide such a system which incorporates a daylight saving time adjustment mechanism to correct any recorded event times which occur during periods when daylight saving time is in effect; to provide such a system in which time is kept internally as standard time and wherein daylight saving time

adjustments are made only prior to the display of an event time, if necessary; to provide such a daylight saving time adjustment mechanism which is applicable to a great variety of time keeping devices; to provide an event clock incorporating such a daylight saving time adjustment mechanism which is adaptable to record event times in a great variety of applications; and to provide such a system which economical to manufacture, durable and precise in operation, and which is particularly well adapted for its intended purpose. Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.

Brief Description of the Drawings

Fig. 1 is a top plan view of a time of a first embodiment of an event clock embodying the present invention with a front cover shown separated. Fig. 2 is a fragmentary front elevational view of the event clock with the front cover removed and illustrating numerals representing a time of day. Fig. 3 is a view similar to Fig. 2 and illustrates numerals representing a month and date.

Fig. 4 is a simplified block diagram of a circuit for a event clock for detecting tilting in which the display is only updated when a tilting operation occurs. Fig. 5 is a simplified block diagram of a second embodiment of the event clock in which a tilting operation causes a resetting and restarting of the clock to indicate the length of time since the occurrence of the tilting operation. Fig . 6 is an enlarged fragmentary front elevational view of a preferred embodiment of an event clock module with daylight saving time adjustment. Fig. 7 is a simplified block diagram of the preferred event clock in which events are detected by the operation of magnetic switches and which incorporates a memory display switch to cause the display a plurality of previously recorded event times. Fig. 8 is a flow diagram illustrating a main routine of a program executed by the preferred event clock. Fig. 9 is a flow diagram illustrating a timer interrupt service routine of the event clock program. Fig. 10 is a flow diagram illustrating a display interrupt service routine of the event clock program including a daylight saving time adjustment process. Fig. 11 is a flow diagram illustrating an event interrupt service routine of the event clock program for recording the time of occurrence of an event.

Detailed Description of the Invention

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Referring to the drawings in more detail: The reference numeral 1 generally designates a first embodiment of an event clock embodying the present invention in the form of a time of tip-over indicator unit. The unit 1 is illustrated attached to a structure 2 which is subject to tilting, such as a refuse container. Referring to Fig. 4, the unit 1 generally includes clock circuitry 3 operating as a digital clock, a tilt switch or mercury switch 4, a battery 5, and a digital clock display 6. Whenever the structure 2 is tilted through certain orientations, the switch state of the tilt switch 4 is changed, and the change of switch state is detected by the clock circuitry 3 which causes an alteration of the clock display 6 in such a manner as to indicate the time of occurrence of the tilting operation. The unit 1 is housed within a case 10 which seals its circuitry from humidity and prevents tampering with the indicated time. The case 10 is preferably formed of a high

impact and durable plastic which does not deteriorate from exposure to sun, rain, ice, and excursions of temperature. A removable front cover 11 is provided to protect the display device 6. The cover 11 may be removed to read the time and date indicated on the display 6. The circuitry 3, mercury switch 4, battery 5, and display 6 are all positioned within the case 10 in such a manner that they cannot be removed without indicating that tampering has occurred. Alternatively, provisions may be made for periodically replacing the battery 5 by the manufacturer. The illustrated case 10 includes side flanges 12 or similar formations through which fasteners, such as rivets 14, pass for attachment of the unit 1 to the structure 2. Referring to Fig. 4, the unit 1 includes the clock circuitry 3 which maintains an internal time count, proportions the time count to hours and minutes of the time of day along with the month and date. The circuitry 3 causes the display of indicia representing the time and date, such as numerals, to be displayed on the display device 6. The illustrated display device 6 is preferably a liquid crystal display device. Fig. 2 illustrates hour numerals 16 and minute numerals 17, a time designation 18, "a.m." and "p.m." indicators 19 and 20, and a daylight saving time (DST) indicator 21, and displays the time 5:43 a.m. In Fig. 3, month numerals 22 and day of month or date numerals 23 are displayed along with a "date" designation 24 and displays the date of September 28. Typically, the time display shown in Fig. 2 is alternated with the date display shown in Fig. 3. This simplifies the unit 1 by avoiding the necessity of a selection switch to select the time or

date or a somewhat more expensive display device 6 which is capable of displaying the time and date simultaneously. In Figs. 2 and 3, the indicia shown in phantom represent nonactivated portions of the display device 6. The tilt switch 4 is preferably a mercury switch which is mounted in the case 10 in such a manner that when the display device 6 is in a vertical plane and oriented for reading left to right (as shown in Figs. 2 and 3), the tilt switch 4 is in a first switch state. The unit 1 is affixed to the structure 2 in such a manner that the tilt switch 4 is normally in the first switch state. When the structure 2 is tilted from a normal first orientation to a second orientation, the tilt switch 4 assumes a second switch state complementary to the first switch state. When the structure 2 is tilted back to the original first orientation, the transition of the tilt switch 4 from the second switch state to the first switch state is detected by the clock circuitry 3. The clock circuitry 3 responds to this transition by causing the updating of the time and date displayed on the display device 6 to the time and date when the transition occurred. The updated time and date remain on the display device 6 until the next occurrence of the switch state transition. The first switch state may be an open state or a closed state, depending on the electrical polarity or logic state requirements of the clock circuitry 3, and the second switch state is the opposite state. The clock circuitry 3 may be a specialized clock/calendar circuit or chip which maintains the time of day and date internally and only updates the display upon detecting a selected voltage transition. The illustrated

clock circuitry 3 is a microprocessor which is mask programmed to perform timekeeping and display updating functions. The microprocessor 3 is powered by the battery 5 and has an interrupt terminal 26 which is connected to the battery 5 through the tilt switch 4. The microprocessor 3 is similar to many conventional types of microcontrollers and has an internal crystal controlled time base generator outputting a timed pulse train which increments an internal counter. Registers within the microprocessor 3 store values representing both the internal and displayed hours, minutes, month, and date. Additional registers store flags to control the a.m. and p.m. indicators 19 and 20 and the time and date designators 18 and 24. The correct time and date are set by the manufacturer, and thereafter the unit keeps time internally. A time count from the internal counter is proportioned into minutes, hours, date, and month; and the registers storing data representing these variables are appropriately updated as the time count proceeds. The time count is also proportioned in such a manner as to time the alternation of the time and date displays. When the second to first switch state transition occurs, the voltage transition on the interrupt line 26 causes the microprocessor 3 to copy the contents of the registers storing the internal values of the minutes, hours, date, and month to the respective display registers for these values. These values are then written to the display device 6. In the meantime, the microprocessor 3 continues its internal timekeeping functions.

The microprocessor 3 is preferably implemented using a low power consumption solid state technology, such as low power complementary symmetry metal oxide semiconductor (CMOS) technology. This, in combination with the inherent low power consumption of the liquid crystal display device 6, provides for very low power drain from the battery 5. The battery 5 is preferably a lithium battery which, in some low power applications, has a useful life in excess of five years and, in some cases, as long ten years. Thus, it is possible in practice that the battery 5 will never have to be replaced. It is also possible that the cost of labor and shipping involved in having the battery replaced by the manufacturer will not be significantly less than the cost of simply replacing the unit 1 with a new one, particularly if the units 1 are bought in quantity. The structure 2 which is subject to tilting has been identified as a refuse container which is emptied by tilting. The unit 1 may also have application in indicating the time of occurrence of other types of actions. For example, in security applications the unit 1 may be used to indicate the last time of occurrence of the turning of a door knob or gate latch, the actual opening of a door, or similar types of actions. The date and time of occurrence function of the unit 1 may also be indicated in response to actions other than those which require tilting operations. Fig. 5 illustrates a second embodiment of the time of event clock according to the present invention. The unit 30 is simpler in implementation and operation than the unit 1. The circuitry of the unit 30 includes a battery 31, a tilt switch 32 such as a mercury switch, and a fairly

conventional clock/calender/display module 33. The clock module 33 maintains a time count and contemporaneously updates a display portion to show the current time, in the manner of a conventional digital clock or watch. Indicia representing the time of day may be alternated with indicia representing the month and date. The battery 31 provides operating power to the clock module 33 through the tilt switch 32 when closed. When a structure to which the unit 30 is attached is tilted, the tilt switch 32 is opened, thereby shutting down the clock module 33, and thereafter closed, thereby restarting the clock module 33. Whenever the clock module 33 is restarted, it resets to a given time and date, such as twelve o'clock midnight on January 1, and begins updating the display portion therefrom. In the unit 30, this mode of operation can be used to indicate the elapsed time since the structure 2 was last tilted. In the case of a refuse container emptied by tilting, this can indicate when the container was last emptied by subtracting the amount of time represented by the display from the current time of day. While the time of tip-over indicator unit 30 does not display its time indication in quite as convenient a format as the unit 1 does, it is adequate for the purpose intended. The unit 30 may be enclosed in the same type of case 10 as the unit 1 and affixed to the structure 2 in the same manner as the unit 1 is. Figs. 6 and 7 illustrate a third, and preferred, embodiment 40 of the event clock of the present invention. The event clock 40 is similar in many respects to the clock 1. The clock 40 is sealed within an enclosure 42 having

side flanges 43 for attachment of the clock 40 to a structure 44, such as a wall within an access panel of a portable toilet or the like, as by rivets 45. The clock 40 includes a display device 46, preferably an LCD display, which is capable of displaying event time numerals 47, event date numerals 48, an a.m. indicator 49, a p.m. indicator 50, a low battery (BAT) indicator 51, and a daylight saving time (DST) indicator 52. As illustrated, the a.m. indicator 49 is shown in phantom while the p.m. indicator 50 is darkened to indicate that the time shown is after noon. The battery indicator 51 remains unseen until internal circuitry within the clock 40 determines that the level of a battery 53 which powers same has dropped below a selected level, indicating that unless the battery 53 is replaced, the clock 40 will become unreliable and eventually cease to function. Preferably, the battery 53 is a long life type of battery, such as a lithium battery and is not replaceable. As such, when the battery 53 becomes exhausted, the entire clock 40 should be discarded and replaced. Fig. 7 illustrates the major components of the clock 40. The clock 40, similar to the clock 1, is implemented as a microprocessor 55 programmed to function as a specialized digital clock. The processor 55 includes a master clock circuit 56 which forms the time base of the clock 40, read- only memory (ROM) 57 which stores an operating program 58 for the processor 55, and read-write memory (RAM) 59. Preferably, the microprocessor 55 is a one chip microcomputer or microcontroller. The microprocessor 55 may be implemented from a variety of commercially available

microcontrollers, such as a National Semiconductor COP 842C. The master clock 56, ROM 57, and RAM 59 are preferably internal to the microprocessor 55. The processor 55 includes integral, programmable input/output ports (not shown) to which an event switch 60 and a memory display switch 61 are connected. The switches 60 and 61 are illustrated as magnetic reed switches which may be actuated to change state by the placement of magnets 62 and 63 in close proximity thereto. Actuation of the event switch 60 causes the clock 40 to record the instantaneous event time and date and, eventually, display same. Actuation of the memory display switch 61 causes the clock 40 to cause a history of recorded event time and dates to be displayed cyclically, as will be detailed below. When the manufacturer of the clock 40 is ready to ship a clock 40, a fresh battery 53 is installed, and the case is sealed. The clock 40 is then set for the correct time and date, including an indication of the current year. The setting time and date are entered by connection of a setting computer 64 to a serial port (not shown) of the processor 55, and data indicating the correct time and date are read in serially, as indicated at 64 in Fig. 7. Normally, once the clock 40 is set, it continues to run internally until the battery 53 is exhausted. When the clock 40 is set, it must be set for the same time zone in which it will be installed. Alternatively, the buyer of the clock 40 may be provided with a setting computer 64. Setting the clock 40 in this manner avoids the necessity of providing setting switches, which might be tampered with when the clock 40 is installed in the field.

A calendar is kept within the clock 40 in terms of "serial dates" which are numerical counts of days since a selected calendar starting date. Each year thereafter can be determined by counting the number of days in a year from the starting date. Leap years can be accounted for by a year determination from the starting date. A daylight saving time (DST) status table 65, or table of DST change dates, is stored in the ROM 57 and has serial date entries for the start and end of daylight saving time for each of a plurality of years. Normally, the table 65 need not include years much beyond the expected life of the battery 53. In general, time within the clock 40 is kept internally in terms of a time count. Event times are stored in the RAM 59 as "date/time stamps" 66 which includes a numerically expressed serial date and a time count. Prior to displaying an event, the serial date of the date/time stamp is converted to a month and date, and the time count thereof is converted to hours and minutes, a.m. or p.m. As will be detailed below, if the date/time stamp was recorded when DST was in effect, the DST indicator 52 is activated. Figs. 8-11 illustrate important routines of the operating program 58. Fig. 8 illustrates a main routine 75 of the program; Fig. 9 illustrates a timer interrupt service routine 76; Fig. 10 illustrates a display interrupt service routine 77; and Fig. 11 illstrates an event interrupt service routine 78. Referring to Fig. 8, when the battery 53 is initially installed and the clock 40 is powered up, the processor 55 executes a normal power-on or reset 80 routine. When the setting computer 64 is connected to the processor 55 and the time and serial date are read in, the

processor 55 initializes time count and serial date variables stored in the RAM 59, as shown at 81. Thereafter, the processor 55 executes a "wait for interrupt" instruction 82 until an interrupt is asserted. Time is kept within the clock 40 by counting pulses or from the master clock 56, which is an oscillator in reality. A counter (not shown) within the processor 55 counts the pulses and provides a timer interrupt 85 after selected pulses have been counted. This causes the time count to be incremented, 86. The time count is tested at 87 after each incrementing to determine if a time count overflow has occurred, signalling the end of a day. The master clock 56 is very accurate, but not absolutely precise. Therefore, an adjustment of the time count is made every so many days to minimize the cumulative error. In the clock 40, the timer count is adjusted a selected number of counts, determined empirically for a given master clock frequency, once every eight days. At 88, a test is conducted on the serial date to determine if an adjustment is to be made. If so, at 89 the time count is decremented a selected number. If the timer limit 87 has been reached and an adjustment 88 is not to be made, the serial date is incremented at 90. If the timer limit 87 has been reached, and a timer adjustment 89 has been made, the serial date will be incremented during an ensuing timer interrupt, since the adjustment reduces the time count. After a timer adjustment 89, a serial date increment 90, or a non-overflow of the timer limit 87, the timer interrupt routine 76 ends with an interrupt return 91, and the processor 55 resumes a wait for interrupt mode 82.

Jumping ahead to Fig. 11, operation of the event switch 60 causes an event interrupt 95 to occur. This causes the processor 55 to read the current serial date and time count and overwrite the oldest date/time stamp 66 in the RAM 59, as shown at 96. The event interrupt routine 78 ends thereafter by an interrupt return 97 to the wait for interrupt mode 82. The newest date/time stamp is the latest event time, but need not be displayed immediately although it alternatively could. Operation of the memory display switch 61 causes a display interrupt 100 to occur. This causes the processor 55 to display date and time respectively of each of the date/time stamps 66 stored in RAM 59, starting from the oldest to the latest and pausing a selected length of time between each. The latest recorded event time is left on the display 46. Before each date and time is displayed, the serial date is tested at 101 to determine if it occurred during a period when daylight saving time was in effect. This is accomplished by comparing the serial date with the table 65 to determine if the serial date falls within the DST status change dates within a given year. If so, a DST flag is set, and the time count is adjusted by an amount equal to an hour at 102. If the recorded event time occurred after eleven o'clock at night, adjusting the time count might be thrown into the next day. This is determined by an overflow test 103. If a time count overflow occurs, the serial date is incremented at 104. Thereafter, the time count is converted to hours and minutes at 105; the serial date is converted to a month and date at 106; and both are displayed respectively

as the time numerals 47 and date numerals 48 on the LCD display 46. One of the indicators 49 and 50 is activated, as is appropriate, and the DST indicator 52 is activated if the DST flag was set at 102. The display and pause activity 107 completes a display loop 108. When the pause times out after about five seconds, a test is run at 109 to determine if additional date/time stamps 66 need to be displayed, by checking a counter which is decremented at the end of each display loop 108. When all the date/time stamps 66 have been displayed, the display routine 77 returns from the display interrupt at 110 and control returns to the wait for interrupt 82. The timer interrupt routine 76 is superior to the display and event routines 77 and 78 since service of the timer interrupt routine 76 is required to maintain the precision of the clock 40. Thus, the timer interrupt 85 may interrupt processing of the routines 77 or 78 to update the time count, which are returned to thereafter. The routines 75-78 illustrated in Figs. 8-11 are exemplary, and other methods of operating the clock 40 are contemplated. Such other methods are considered as equivalents to the routines 75-78 illustrated and are intended to be encompassed by the present invention. It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.