Yes, at midnight UTC your blog's "day" ends, too, according to WP Stats.
BTW: UTC = PST - 8 hours (so, UTC rollover occurs at 4:00pm PST)
Also, note that under Options > General settings you'll see that WP itself also operates under UTC, which you can offset by whichever number of hours you're at. This is somewhat different from some other plugins and scripts, that operate at the server's timezone - from which you'd have to set the proper offset in hours relative to your local time. Finally: UTC doesn't recognize / observe daylight saving time (DST), so during DST, UTC = PST - 9 hours
Of course, there are some other stats plugins that do work with the same offset you use (set) for your entire WP blog, under Options > General.