Apple continues to deal with problems with the clock on the iPhone -- this first cropped up back in November, when clocks were turned back an hour to standard time. Some issues were still popping up in January, and now this weekend, as clocks were turned forward for Daylight Savings time in the US, some users were still having issues of varying kinds, from clocks changing when they weren't supposed to, to alarms not going off when set up correctly. Some users are even reporting the time changing just fine, and then inexplicably switching back -- one of my friends had a problem like that with his first-generation AT&T iPhone last night.
Unfortunately, since most of the information on the issues is anecdotal, it's hard to pin down exactly what's happening, but it seems to be something going on between the phone's software and a connection to the local provider's time information. Most of the issues are being fixed by flipping Airport Mode on and off (to re-establish the cellular connection) or even just restarting the phone. Alternately, you could just set the time yourself manually, though having a phone that sets its own time is obviously a lot more convenient.
At any rate, it looks like Apple is continuing to deal with this ongoing issue. Do make sure to check your phone and get it right -- you wouldn't want to miss Pi Day today, after all.
Apple iPhone 6s