Sunday will end at 23:59:59 (11:59:59 pm). At 00:00:00 (12:00:00 am) (midnight), a new day (Monday) starts.
I think you are technically correct. But I don't think CO even knows. I just completed my reservation that was supposed to be held until Midnight Monday. The agent thought that would mean until the end of Monday, but I think it's actually (as you say) the beginning of Monday at 12:00am.