38E/C, 12C, 12CP date arithmetic bug Message #1 Posted by Katie Wasserman on 19 Oct 2011, 11:28 a.m.
A while back a date arithmetic bug was found during beta testing that affects all the 12C, 12CP, 38E/C and 12C clone calculators ever made. It's been unreported here (or anywhere else as far as I know), it's really obscure and was apparently fixed in the Pioneer series and after.
If you're not aware of this, the 38E/C and the 12C/CP calculators compute both 365 day calendar and 360 day calendar date differences in the X and Y registers respectively. If you take a forward date difference to February 28 both values (365/360) are correct. If you take a reverse date difference the 360 day value is wrong.
For example: 12/31/2007 to 02/28/2008 returns X=59, Y=58; while 02/28/2008 to 12/31/2007 returns X=59, Y=57 and that 57 is wrong. Pretty clearly backwards in time should be the same number of days as forwards in time.
While this is extremely unlikely to ever occur during normal usage, it's useful as a calculator forensic. You'll find that the 12C/12CP clone calculators  the Aurora FN1000, Compucessory CCS 28956 and Victor V12  have the same bug. Prior to the 38E/C, the 92 offered 365/360 date arithmetic, but that gives different answers using 360day calculations.
Katie
Edited to correct 3/31/2008 to 03/01/2008, but that was the wrong date too, the correct date for the example is 02/28/2008.
Edited: 19 Oct 2011, 10:05 p.m. after one or more responses were posted
