When Does a Day Start?

For the "x per day" features under Platinum or Gold membership, when does the day start? Is it when you pay your subscription (ie, different for all) or is the same time as the daily puzzle goes up (ie, the same for all)?

If it the day is fixed for everybody, what difference would that really make-- except on one day?

You do 25 tactics trainer problems at noon one day and then you get up at 3am and do another 25. Yes, you beat the system that day by getting 50 problems in 24 hours.

But you can't get another 25 until 3am the following day.

Thanks. I asked simply because, if it's related to the time you sign up, then I'll time it to rollover just before I like to do tactics problems (in the morning before I go to work). It wasn't a matter of gaming the system or trying to get more problems in a 24-hour period.