E-assignment submission system made my effort go to nothing My assignments deadline was due to March 13, 00:00. I uploaded my assignment half a day earlier, but did not confirm it, because instructions said we do not have to (last uploaded version always gets saved). But, stupid me, I confirmed my assignment after the deadline, without uploading new version, and that overwritten my original submission time. 

Now it looks like I was late with my assignment. By 15 minutes.

What to do in such situations? I was thinking of writing to a teaching asistent and explaining the situation.
You can explain the situation to your professor, and he/she may understand the situation and confusion. However, if what you say is true and how your last submission overwrote your original submissions - assuming your original submission is now lost, then it looks like you submitted nothing and it looks like you missed your deadline. Your best bet is to contact the professor. Perhaps he/she can see your previous submission details.