Usually if I have the timetables available, and if I know the exact day of first run from each end, I update the exact day.
If I do not have information about the first run, I update the same date for both Up and Dn trains.
I am fine with either approach. If the other monitors think that we should update the same date for both Up and Dn trains, then I can go ahead and change all the inaugural dates I added.