Also keep in mind that the time on your cell phone is broadcast along with the signal. When the signal is working the time on most cell systems is from a GPS and nearly perfect. If the signal is not working ... you may get no time on your cell phone.
As mentioned, GPS time is about the best (most accurate) you can get, but you have to turn on the GPS to get it and that sucks precious batteries power.
When I envision most survival situations the car is not there too. Or, if it is, it may not be functioning correctly.
When I'm in the outdoors I use my watch to schedule my time, to make sure that sunset doesn't catch me unprepared for nightfall, and even to make sure I get up early enough to get in a good day's work.