Can a player accumulate turns?

[Satan]

Time adds up, to a certain limit. If rollover comes and the amount will put you over 21 hours of unplayed time, it rounds down to 21 hours, and won't increase from there by rollover at all. At 5 minutes per turn, 21 hours is 252 turns, so you've got a bit of leeway.

[Harry Dresden]

No, that is the maximum. If you have 20 hours of turns left at rollover, you will only have 21 hours (or 21 1/2 with Precompletion Pro) after rollover and the rest of the time is lost.

[Ryme]

Just for reference, some of this is explained in the manual a bit, too.