I used to work for a company that wrote medication management software for hospitals. We encountered a problem where it kept scheduling things wrong for some patients, but only occasionally – and we knew it couldn’t be a daylight savings time issue because it went by clock time, so if it said “administer at 9am and 4pm” then that was what was done; it didn’t do date/time calculations at all. Except it did: one of our programmers was permanently remoting in from Queensland, which doesn’t do DST, so he assumed you could short-cut the date calculations by taking an existing value for “today at 9am” and adding twenty four hours, and guarantee that it would translate to “tomorrow at 9am”. Those of us who worked in Canberra, which does DST, would never be so foolish. We caught it and made him change it to the equivalent of newdate = DATE(olddate.year, olddate.month, olddate.day+1,olddate.hour,olddate.minute,0) and then all was well.

So even when you don’t need to worry about time, you need to worry about time.