(With apologies to John Sutherland)

How old is the Doctor? That’s easy. He’s 1103. He said so, in The Impossible Astronaut. Case closed, as Patricia Cornwell might tiresomely say.

Except, while that age is entirely consistent with the evidence we have in the Matt Smith episodes, it is not consistent with other dates established elsewhere in the series:

• The second Doctor tells Victoria that he is 450 years old in The Tomb of the Cybermen.
• The fourth Doctor is “something like” 750 years old in Pyramids of Mars, Romana says he is 759 in The Ribos Operation, and he’s about the same in The Leisure Hive.
• The sixth Doctor says he is 900 years old in Revelation of the Daleks.
• The seventh Doctor says both he and the Rani are 953 in Time and the Rani.
• The ninth Doctor claims he has 900 years of experience in Aliens of London.
• The tenth Doctor states he is 903 years old in Voyage of the Damned and 906 in The End of Time.
• The eleventh Doctor is 907 in Flesh and Stone.

From the onscreen evidence, then, the Doctor ages 300 years between his second and fourth incarnations, 140 years between his fourth and sixth incarnations, and then gets 50-odd years younger between his seventh and ninth lives. Which is odd, to say the least.

For anyone who cares about this kind of thing, then, the “900 Controversy” (as it’s been melodramatically labelled) is one of the most egregious discontinuities between the classic and new series. But need it be? Can the different dates be reconciled?

The only dates that are hard to square are the ages quoted by the sixth and seventh Doctors. If you really want to, it is not difficult to posit a long period of adventures, for example, for the second Doctor after The War Games (for Season 6B purists), or for the third Doctor after The Green Death, to explain the 300-year jump from 450 to 750.

However, Big Finish audios aside, it’s harder to see when the fourth, fifth or sixth Doctors might have aged 140 years. The BBC website on The Doctor’s Age suggests between The Leisure Hive and Meglos, but this seems unlikely, given the serialised nature of Season 18, the fact K9 is still sea-damaged and Romana’s still wearing her Leisure Hive beach outfit at the start of Meglos. The evidence suggests that the fourth Doctor is about 760 when he regenerates. Which means, for the Doctor to gain 140 years by Revelation of the Daleks, either Nyssa ages at a very slow rate between Time-Flight and Arc of Infinity, or Peri gets dropped off somewhere for a long time.

Nevertheless, the seventh Doctor says he is 953 – and unless it’s a particularly obscure joke, which seems unlikely given it fits with the sixth Doctor’s previous declamations, this does not tie up with the tenth Doctor’s stated age of 903.

The Time War then? Suggesting that the Doctor genuinely did de-age 50 years during the Time War is possible, but, like anything that relies on the Time War for justification, seems a bit lazy. And arguing that the Doctor is somehow ageless or that he has forgotten (as Steven Moffat has) is unsatisfactory too, because he is so specific about it on so many occasions; it’s a plot point in Time and the Rani, and the punchline to a joke in The Ribos Operation, both of which depend on Time Lords other than the Doctor stating his age. And both Moffat, Russell T Davies and Graham Williams seemed keen to maintain the consistency of the Doctor’s age, having him get about a year older each series.

So how else can we resolve the “900 Controversy”? The only time in the classic series that we can see the Doctor almost certainly giving his age in Earth years is in The Tomb of the Cybermen, where Troughton makes a great show of having to work it out for Victoria. Arguably, on every other occasion in the classic series he’s talking in Gallifreyan time. That would definitely make sense of his various conversations with Romana, and the 953 comment in Time and the Rani, as both Romana and the Rani are exceedingly unlikely to measure their ages in Earth years. If you follow this logic, then in the classic series the Doctor is about 450 Earth years old, and in the new series he is 900-odd Earth years old – there being no Gallifrey to measure years by any more.

If we take that one step further (and why not, given we’ve got this far), let’s say 450 Earth years equals about 740 Gallifreyan years. That means one Earth year lasts 1.6 Gallifreyan years. Which would mean in Pyramids of Mars the Doctor is actually about 460, and in Revelation of the Daleks he’s about 550. That still means he ages 90 years at some point after The Leisure Hive, but otherwise fits the bill. And the big, 300-plus jump between Time and the Rani and Aliens of London can be explained by the adventures of the seventh, eighth and ninth Doctors during the New Adventures, EDAs, Big Finish plays and the Time War. It’s certainly easier to believe than 300 years between The Tomb of the Cybermen and Pyramids of Mars.

But that still leaves the question why Russell T Davies didn’t just make the ninth Doctor 1000 years old to avoid fans jumping through hoops to reconcile two dates. It’s hard to believe he wasn’t aware of the discontinuity, and he’s a thoughtful enough writer not to throw in a random number for the sake of it. The answer’s simple. There’s something mythic about 900 years – it rolls off the tongue, and has a weight to it that the more prosaic 1003 doesn’t. It’s an Old Testament kind of a number, a Yoda kind of a number. Davies’ Doctor is 900 years old because he’s mythic: the lonely god, the last of the Time Lords. He has to be 900. He couldn’t be any other age.