How to reset the clock?

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

You can download a manual from http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

If you can't find yours, you can download a new copy of the manual from the manufacturer's web site at http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

You can download a copy of the manual from the manufacturer's web site at http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

You can download a manual from http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

If you want to set the time, the button that has a picture of the calendar and clock (just to the right of the arrow key) until it shows the time. Then press and hold the date/time set button (just tor the right of the red minus button) for three seconds until it begins to blink. Then use the arrow key and enter appropriate digits and the am/pm button for am or pm. If you need to set the date, press the calendar/clock button again until it shows either the date or the time, and then hold the date/time set button down for three seconds until it begins to blink. Then use the arrow key and enter appropriate digits.

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

You can download a manual from the manufacturer's web site at http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

Press and hold the ClockDate Set button for three seconds until the display shows the date with the first digit blinking. Enter a two-digit month, a two-digit date, and a four-digit year, followed by the ClockDate Set button.

Press and release the ClockDate Set button. Press and hold it for three seconds until the display shows the time with the first digit blinking. Enter a four-digit time, press AM/PM as appropriate, then press the ClockDate Set button. Press CE/C to finish and return to calculation mode.

You can download a manual from http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

Download the user manual (P23-DH III) from here:
http://rapidshare.com/files/181603396/P23DHIII.PDF.html

I don't think the P23-DH has a calendar/clock mode. I have found the instructions for setting date and time for the P23-DH V. If your calculator does not have [Calendar/Clock], [Time Set], and [AM/PM] keys, you cannot set the date and time.

To set date:
1. Press the [Calendar/Clock] key once
2. Press and hold the [Time Set] key for three seconds
3. Enter the date in MMDDYYYY format
4. Press the [Time Set] key
5. Press the [C/CE] key

To set time:
1. Press the [Calendar/Clock] key twice
2. Press and hold the [Time Set] key for three seconds
3. Enter the date in HHMM format, press [AM/PM] key if necessary
4. Press the [Time Set] key
5. Press the [C/CE] key

Slide the on/off switch all the way to the left to the "Rate Set" position. Now enter the appropriate tax rate. When entered, slide the switch to the desired "On" or "Print" position and CE/C before your first calculation. You should now be all set to go.