fix datetime

[ejovrh]

at the moment the time is obtained by GPS GPRMC sentence.
the timefield in there is GPS-time, not UTC.

fix the code so that proper UTC is displayed and not GPS time.
furthermore, fix the code so that displayed time is in accordance with the UTC setting in the eeprom and that the displayed date is in accordance to 235959 -> 000001 change (next day should be visible in the datum)