In a somewhat similar scenario, we had some discussions about what "Friday midnight" means. So just to be super clear, we finally put "Thursday, 11pm".
Interesting cultural differences. In other places if you submit earlier than the deadline they assume you didn't care about it enough to use all available time to make it as good as possible.
As a range or interval specifier, many (most?) non-programmers will assume the interpretation of “midnight” that favours them in any subsequent dispute.
In practice this often means that “from midnight on Monday to midnight on Tuesday” is a 48-hour interval so far as consumers are concerned. I recommend advertising things like cut-off times as “11:59pm” and friends, when possible.
Also, my time formatter turns “12:00” into “12 noon” following weary experience of people who confuse 12:00 with midnight.
I told them that if they wanted me to submit it on Thursday, they shouldn't've set the boundary for Friday at midnight. That didn't go over too well.