This file defines starting year and day in game time and format of date.
In BG2 ToB we have:

2DA V1.0
0
                        VALUE
STARTTIME               878400
STARTYEAR               1368
NORMALDAYMONTHFORMAT    15981
SPECIALDAYMONTHFORMAT   16001

where:
first number determines day in given year when game is started (as defined in GTIMES.IDS ). In our case it is 122 day of this year ( 878400/7200 = 122 )
second number is just the starting year and next two numbers are strrefs to dialog.tlk filled by tokens that show date (15981 is for example "<DAY> <MONTHNAME>" )