The automatic setting of the date and time is served up by a carrier protocol (not the internet time servers as used by your computer). If you're getting your time resetting whenever you perform a soft or hard reset, then I'd suggest turning off the "update automatically" box in the settings. Sometimes, I've seen that this doesn't work and the time/date are still set wrong after a hard reset (I'm not sure if this is fixed in WP8.1 or not).
But, after a hard or soft reset, I habitually check the date/time and reset if necessary.