Why would a 23:59 deadline not be inclusive?

Just had to submit something at school. Thinking I have plenty of time, I took it. I wanted to write a note along with the submission, so I took my time until like 23:59:20 when I hit submit.

The fuck you mean submission not possible past deadline? Why is it not the next day 0:00 if exclusive?

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The difference from Time.is was +0.015 seconds (±0.067 seconds).

  

In HS I used to do such timing for comedic purposes, guess it won't work here.