One thing that leaps to mind when I see people doing date and time arithmetic is that ,sadly, not all days have 86400 seconds (24 hours). When dealing with local times, there are such things as 23 and 25 hour days. In Spring you'll get one 23 hour day during the standard-to-daylight-savings switch. In Autumn you'll get one 25 hour day during the DST-to-standard time switch.

You should probably take Corion's advice and have a look at that node he recommended: How do i find yesterday's date. I thought it was pretty good.