At the precise moment when the name tab for the sim baby appears, two things are checked - how far are we into the current season, and what time is it?
In this particular case, we're in the beginning of spring, which is March. (Middle = April, End = May, obviously)
So - baby Lisa is born in March.
Then look at the time, the minutes say 36. If the minutes are higher than the maximum date for the current month (March 31, April 30...), the max date is subtracted from the minutes
In this example: 36 (minutes) - 31 (max date of March) = 5
Which means Lisa's birthdate is the 5th of March.
The year is decided from the cycle we play. Midnight is year end, while 2 am, when all guests leave, is rotation end. That means in this case that Lisa was born in '93, even though the rotation officially ended after '92.
Townies get their birthday on the date they are first presented on this blog. So if a new townie was introduced today, his birthday would be March 2nd. Year is decided compared to the sim he moves in with.