I don't have the manual for my canon p23-dhv calculator. How so you set the date?

I do not think that it can do constant operation calculation (at least none is described in the manual). Here is the link to the manual from Canon.
Just in case the link above is not activated upon clicking it, here it is expliciltly
http://downloads.canon.com/cpr/software/calculator/P23-DHV-%28USA-HWB%29.pdf

See the instructions at the beginning of the manual: http://downloads.canon.com/cpr/software/calculator/P23-DHV-(USA-HWB).pdf

P23-DHV:
http://downloads.canon.com/cpr/software/calculator/P23-DHV-(USA-HWB).pdf

P23-DHV G:
http://www.usa.canon.com/app/pdf/calculator/P23-DHV_G_USA_GB.pdf

The calendar and clock setting section is the first thing in the user guide at
http://downloads.canon.com/cpr/software/calculator/P23-DHV-(USA-HWB).pdf

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

Follow the illustrated instructions in the manual available at the manufacturer's web site
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=346&modelid=15431

http://www.usa.canon.com/cusa/support/office/calculators/portable_palm_printers/p23_dh_v#BrochuresAndManuals

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

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.

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