You don't mention what GPS, so I assume that you are thinking about a GPS with the NMEA protocol. There are other protocols too, such as SiRF Binary.

The NMEA string with just date/time info is ZDA string. But not all GPS have it.

There are no easy standard way to control what strings your GPS emits, and how often. Get the GPS and get the manual. Settle for any of the strings it emits. At least one of the strings with date/time info will be emitted once/second. A common string to look for is the $GPRMC string. It's a good general-purpose string with a lot of goodies it in, and most GPS supports it.