I'm in UTC-7, and the new day seems to occur at 4:00 PM my local time.The system counts a new day as starting three hours after midnight, as this is a time most people are likely to have finished their day's studying.
Using browser developer tools, I did notice a POST to https://www.maorma.net/Practice.aspx/UpdateTimeZone with my time zone, so I assume the intention is to account for local time zone.
Additionally, if I happen to be practicing through the start of a new day, the previous day's practice time is carried over into the new day, essentially double-counting the time from the previous day.
None of this really matters besides making my statistics inaccurate, but I thought I'd mention it here in case it was just some typo in whatever is handling the UpdateTimeZone URL.
Of course, thanks for creating and continuing to run this site. It's been part of my daily routine for over a year now.