Is Midnight AM or PM?
Midnight is 12 AM by convention — and it belongs to two dates at once, which is why deadlines written at midnight go wrong. Here is how to write it clearly.
By convention, midnight is 12 AM — the start of the new day. But midnight sits on a boundary between two dates, and that is a real ambiguity rather than a thing people simply get wrong.
The Convention
Midnight begins the day. So midnight between Monday and Tuesday is 12 AM Tuesday, not 12 AM Monday. In 24-hour notation it is 00:00 Tuesday.
That is the rule most systems follow. It is also the rule most people do not have in mind when they read "midnight Monday".
Why Midnight Belongs to Two Dates
Every other minute of the day belongs to exactly one date. Midnight is the instant one date ends and the next begins, so a plain "midnight Monday" has two defensible readings:
- 00:00 Monday — Monday is just starting, 24 hours before the other reading
- 24:00 Monday — Monday has just ended, which is the same instant as 00:00 Tuesday
Both are used in practice. Transport timetables and legal documents often mean the second; software and calendar apps almost always mean the first.
Where This Actually Bites
- Deadlines — "submit by midnight Friday" is read as Thursday night by some and Friday night by others.
- Insurance and contracts — cover starting "at midnight on the 1st" may or may not include the 1st.
- Opening hours — "open until midnight Saturday" usually means into the early hours of Sunday.
- Date filters in software — a range ending "midnight on the 31st" typically excludes almost the whole of the 31st.
The Fix: Never Write Midnight for a Deadline
| Ambiguous | Unambiguous |
|---|---|
| Midnight Friday | 23:59 Friday |
| By 12 AM Monday | By Sunday 23:59 |
| Starts at midnight on the 1st | Starts 00:00 on the 1st |
23:59 is the single most useful substitution. It is unmistakably the end of the stated day, and no one has to decide which side of the boundary it falls on.
In 24-Hour Time
The 24-hour clock has both notations available: 00:00 starts a day and 24:00 ends the previous one. They refer to the same instant with different dates attached. Most software accepts only 00:00. [0000 vs 2400 military time](/blog/0000-vs-2400-military-time) covers the distinction in full.