I did some tests this week with my K100D Super and everything works well on my side
Test 1: same day
- insert AA batteries
- update the date
- remove AA batteries few minutes
- check the date
Test 2: 2-3 days
- check the date
- remove AA batteries few minutes
- recheck the date (no change)
However, I remember when I don't use my camera and remove the battery for a long time (like 1 month or more), the date came back to the default time. So I assume that there is a small battery in the Pentax K100DS. Probably that yours is almost dead...