When my advisor told me that it takes a semester just to graduate, she seriously wasn't kidding.
Here's the math:
To graduate this December, I had to have everything signed and turned in on December 6th. Doesn't sound so bad, right?
But that basically means I have to defend in November.
And my college/department has a mandatory pre-defense which must be a MONTH before the real defense.
/*Note on the pre-defense: I am not sure how many universities require a pre-defense. It has some pros and cons.
Pros: you have everything in a state of readiness a month before you really need to, and if anything is glaringly horrible and you might not graduate because of it, you find that out before your defense and likely before you tell everyone you are going to graduate. The pre-defense is private, so your presentation is critiqued and likely better for the public real defense, which is to everyone's benefit.
Cons: you have to have everything ready a month earlier that you really need to. Your committee might use it as an excuse to tell you to do extra things because you have a month. Your committee has to sit through basically the same talk twice, and it is sort of a waste of their time.
end of note*/
Thus the pre-defense must occur in October.
And your committee needs to read the dissertation before you pre-defend it, so you really need to give it to them 2 weeks before the defense.
This means that to graduate in December, you basically need your dissertation in a state of readability and relative completeness by the end of September! And since the semester starts at the beginning of September, you have essentially 4 weeks of the fall semester to work on your dissertation.
And the same goes for a spring graduation.
If you want to graduate in May, you defend in April, pre-defend in March, and have have everthing turned in by the end of February.