In the last of BBC Books' Eighth Doctor books, The Gallifrey
Chronicles, Lance Parkin reveals the approximate date of the Doctor's
birth/looming, which is given as follows:

309456/4756.7RE/1213GRT/100447TL

30946 is a date based on the Dating Index mentioned in "The Making of
Doctor Who" by Malcolm Hulke. 309906 was given as the date given for
the Doctor's trial in the War Games. Which would have made him 450 at
the time.

4756.7 RE is a Rassilon Era date based on the system identified in
the TV Movie starring Paul McGann. The date shown on screen was
5725.2, which suggests that the Doctor was 968 years old when he
regenerated (Vampire Science, however, makes him 1,008/9 - a
discrepancy of 40 years).

The 1213 GRT date is drawn from The Gallifrey Chronicles and
means "Gallifreyan Relative Time". What this date means is uncertain,
but I suspect it is the Earth Date relative to Gallifrey.

100447TL is a system used in the FASA Doctor Who role-playing game.
The TL stands for Time Lords, and infers that the date represents the
number of years the Time Lords have existed. Given that that the RPG
provides a fixed chronology that is occasionally at variance with
subsequent dating, this could be used to refer to how many years have
been spent in "Inner Time", ie. outside of the real universe.

Based on Parkin's date, all dates measure roughly the same unit of
time - the Gallifreyan Year - which is, coincidentally, about the
same as that of Earth.