How should "midnight on..." be interpreted?

From what I understand, the word "midnight" is usually interpreted incorrectly.

Midnight is written as "12am" which would imply that it's in the morning. Therefore, it should be at the start of the day. On the other hand, both Dictionary.com and the Oxford Dictionary define it as "twelve o'clock at night".

Some examples:

1. "Midnight on the 10th of December"
2. "Midnight Thursday"
3. "Midnight tonight"

are usually interpreted as:

1. Straddling the 10th and the 11th of December
2. Straddling Thursday and Friday
3. Straddling today and tomorrow

but should they technically mean:

1. straddling the 9th and the 10th of December?
2. straddling Wednesday and Thursday?
3. This is much less clear. Technically is there a midnight "tonight", or is midnight "tomorrow morning"?

What do you think? How should "midnight" be interpreted?

It's a matter of convention, and the informal convention is that "midnight on the 10th" is more commonly the night between the 10th and the 11th. But the term is awfully ambiguous, and people do use it both ways.

(When I've scheduled things with "midnight" deadlines, I always say "11:59pm on the 10th" or something like that, to avoid the ambiguity. From experience, if you don't people will ask which you mean.)

By most definitions, the date changes at midnight. That is, at the precise stroke of 12:00:00. That time, along with 12:00:00 noon, are technically neither AM or PM because AM and PM mean "ante-meridiem" and "post-meridiem", and noon and midnight are neither ante- nor post- meridiem. However, for convenience, most people lump the 12:00:00 time with its nearest neighbour, 12:00:01, which IS AM or PM.

Since the date changes at the stroke of midnight, there is always ambiguity about which date you refer to. Midnight on the 10th technically means at the start of the 10th, but when most people speak they mean it to be at the end of the 10th.

If I said "I'll meet you Friday at midnight" or "I'll meet you Friday night at midnight" you (and most people) would probably interpret both times as the midnight that follows Friday noon. However, there are cases where the first sentence really means "the midnight at the start of Friday".

Since the common usage conflicts with the technical definition, if you want to be totally clear, use other words or other times.

Friday night at midnight

probably will always be interpreted as "Midnight in the night which follows Friday evening".

Midnight tonight

This means (to me) the midnight following today.

11:59 PM Friday

12:01 AM Saturday

These are totally unambiguous.

Another way to avoid ambiguity is to use a 24 hour clock. Midnight between Thursday and Friday is 2400 Thursday, and 0000 Friday.