Understand your point, but equally you could suggest the pot unlock at 00:01.
Pot unlocks at 00:01 31st Dec '18 completely negates any ambiguity of midnight 31st Dec meaning
* 00:00, the minute immediately prior to 00:01 31st December; vs
* 00:00, the minute immediately prior to 00:01 1st Jan (which some people may assume to be ‘midnight’ of Dec 31st.
Whilst I understand 00:00 to mean the am of the given date, it does leave ambiguity.
Edit: Scrap everything I wrote, I just re-read it and your idea is better.
Unlock pot on: “insert date scroll”