If you own a Hewlett-Packard HP-45 calculator and would like to use it as a digital stopwatch and elapsed-time indicator the way the more expensive HP-55 programmable can, there's good news. You can use your HP-45 for these functions, even though Hewlett-Packard makes no mention of the fact in its Operator's Handbook.

To gain access to the clock function in the HP-45, first clear the calculator by operating the gold alternate-function key. Then press RCL and simultaneously press CHS and the digits 7 and 8, or 5 and 4, or 1 and 2. (It is important that these keys be pressed simultaneously.) The display format will now be

To start the clock function, press the CHS key once. Pressing CHS again will stop the count without resetting the display to zero. To reset the display to zero, simply press the CLx key.

When operating the calculator on batteries, you can save power by blanking the last two digits (hundredths-of-seconds) by operating the EEX key. The clock will continue to run without upsetting the count. You can restore the blanked digits by pressing the EEX key again.

Elapsed Time Indicator. The clock can also be used as an elapsed-time indicator for timing and storing the elapsed times of up to nine separate events. The only constraint is that all events must start simultaneously. This function can be quite handy for measurements of physical phenomena, chemical reaction experiments, etc.

While the clock is running, depressing any 1 through 9 digit key stores the displayed time up to that point in the respective register. The clock keeps running and is not otherwise affected by this action. After stopping the clock by operating the CHS key, pressing any of the digit keys recalls the time stored in the respective register.

Note that the STO key, while the clock is running, and the RCL key, after the clock is stopped, need not be operated. The "store" and "recall" functions are automatically executed, depending on whether the clock is running or stopped. (The CLx key should again be pressed after the last readout of stored time to permit the clock to continue from the reading on the display when it is again started.)

When the clock is no longer needed, you can return the calculator to normal operation by pressing ENTER or turning off and then on the power. The latter method is preferred because it clears the contents of the registers automatically. Even so, the registers don't have to be cleared if you wish to store new elapsed-time data. Whenever new data is entered into a memory location, it automatically clears previous data.

While the clock function is in operation, the only keys that have any effect are CHS, EEX, CLx, . (decimal point), and 1 through 9. All other keys, including the basic arithmetic (+, −, ×, ÷) keys, are inoperative.

Accuracy. It appears that the HP-45 was built with the HP-55 in mind and, hence, uses some of the same circuitry contained in the latter. However, while the HP-55 is "trimmed" for the required oscillator accuracy, the HP-45 is not. The result is that the HP-45, although reasonably accurate, will not be "on the nose." If you determine the percentage of inaccuracy by comparing the HP-45 against a known time standard and make a note of the deviation, you can calculate precise times.

Although the absolute accuracy of the HP-45 as a timer may not be equal to that of the HP-55, the time function used during relative measurements - and particularly for storing up to nine elapsed-time measurements - can be very useful indeed. At least you won't have to trade up to a more expensive calculator.

This is a brief note from a July 1976 publication:

A new useage of an established laboratory tool has come to our attention. One of the scientific caclulators on the market today has, as advertised by its manufacturer, a digital clock - the Hewlett-Packard HP-55. However, another of the Hewlett-Packard calculators, the HP-45, contains an unadvertised clock.

The operation of the clock is as follows:

1. To access the clock, push the [RCL] key followed by simultaneously pushing the [CHS], the [7] and the [8] keys. The simultaneity is important and it may take the user more than one try to do it correctly. The user may easily recognize success as the display will change to 00.00 00 00.
2. The clock is started and stopped by pushing the [CHS] key.
3. The [CLX] key will clear the counter. The counter may be cleared whether or not the clock is stopped.
4. Alternnate keying of the [EEX] button removes and restores the tenths and hundredths counters.
5. While the counter is running, intermediate displays may be stored in 10 memory registers by pushing the [0]-[9] keys. (It is not necessary to press the [STO] key.) The stored results may be recalled after the counter is stopped by pushing the [RCL] key followed by the number of the desired register. The display cannot be stored when the counter is not running.
6. Pushing the [ENTER] key restores the calculator to the calculational mode.

   The calculator time equals 0.8847 real time (measured with a standard deviation of 6 × 10^-4). This there about 53 min 4 sec to the calculator hour. Multiplying results by the reciprocal of the above factor gives quite accurate results. The major usefulness is the ability to store ten intermediate times. An experimenter merely need record which register corresponds to which observation and at the conclusion of the timing of the experiment can access the times of the various events.