The days to maturity is from gerimination to harvest, but it is only an average. For instance, my radishes I planted early this Spring were supposed to take 22 days. I got them in 60+. Since then I've been closer to 30. Frankly, when a veggies is done it's done, and not a day before (unless you like baby veggies, hehe).

Here is a link that might be useful: Sinfonian's garden adventure

From what I understand, if the seed is directly sowed, the days to maturity are counted from when the seeds germinate.

If you are planting a transplant, the days to maturity are counted from when it is planted.

But, like Sinfonian says, when a veggie is done, it is done. My broccoli has been in the ground for over two months, with no signs of forming a head but the days to maturity is supposed to be 55 days. Oh well!!