I have decided to start giving my Sims retroactive birthdays. (I currently have about 50 Sims in-game.)
Here is how I'm doing it:
Each day of Sim time is equal to one year. Monday of Week One was 2011 (because I started this challenge in 2011). Midnight on Tuesday, it became "2012." And so on. In most cases I either know what day of the week my Sims were born or can extrapolate based on their age compared to siblings, etc. (The main problem is having the cutoff be midnight instead of 6 p.m., when Sims age up. I might change this later if it's too much of a pain.)
Then I use Random.Org to generate a number between 1 and 12. This is the month in which the Sim was born. For example, I just rolled for Lauren Whitfill and got a 5, i.e. the month of May.
Then I used Random.Org to generate a number between 1 and 31. (If I had rolled April, I would have generated a number between 1 and 30, if February between 1 and 28 or 29, depending on the year. Etc.) I got 14 for Lauren.
Then I used Random.Org to generate a number between 1 and 24 and got 11, but didn't like that because you can tell very obviously from the pictures that Lauren was born at night. So I rerolled a number between 1 and 12 and got 4. For baby Sims that are obviously born during daylight/darkness, I will roll a number between 1 and 12. (Someone who was born in daylight and got a 6 would be born at 6 p.m., someone born in daylight who got a 7 would be born at 7 a.m. It's very convenient that Sims have exactly 12 hours of daylight and 12 of darkness every day.) For Sims who are not obviously born at one time or another or who were not born in-game (i.e. CAS Sims and ex-Townies), I'll roll a number between 1 and 24.
Then I rolled a number between 1 and 60 and got 57.
So, Lauren was born May 14, 2037, at 4:57 a.m.
EDITED TO ADD: Yeah, the cutoff from one year to the next is 6 p.m. I just could not handle trying to calculate relative ages otherwise.