It's definitely tomorrow.
You can think of it this way. You have to realize that it is midnight for a whole 60 seconds. If you think that midnight is still "today", then you would have to agree that it won't be tomorrow until 12:01:00. But midnight itself refers to 12:00:00. That means 12:00:01-12:00:59 will be past midnight, which would imply that the days don't start and end at midnight, but rather a full 60 seconds after.
That's a contradiction with the definition. So either you accept that midnight is tomorrow, or you assume there is a small slice of time every 24 hours that is completely unaccounted for by the calendar, which doesn't make any sense.