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 'On', 'in' or 'at' midnight?
This week we have two questions about the use of prepositions to indicate time.

Phoebe Chiang from Taiwan writes:
We use in for longer periods of time. We also say: 'in the morning', 'in the afternoon' and 'in the night'. Why can't we say: 'in the noon' or 'in the midnight'?

And Marta Fernandez from Spain asks:
When do we use in and when do we use on with dates? We say 'in September', but can we say 'on September the 29th'?

Roger replies:

'at' with time phrases

We use at to specify a particular point in time. Both noon and midnight are very short periods. When the clock strikes twelve, it will be midnight. We would therefore say: at midnight or at noon. Consider these further examples:

  • 'We'll meet you in front of the cinema at a quarter to eight.'
  • 'I have to get up at six thirty on weekdays.'
  • 'I like to spend some time with my family at Christmas and at Easter.'
  • 'What are you doing at the weekend?'
  • He was born at the end of the 19th Century and died at the end of the 20th.'
Note that although both Christmas and Easter last for a few days, we prefer to think of them as a particular point in time and therefore use at when referring to them. 'At the weekend' follows a similar pattern, though Americans would say 'on the weekend.'
'in' with time phrases

As you rightly say, Phoebe, we use in to specify periods of time, parts of the day, morning, afternoon, evening, or for longer periods altogether. Consider the following examples:

  • 'My dad prefers to work in the morning. He's too tired to work in the evening.'
  • 'My granny always has a cup of tea at four o' clock in the afternoon.'
  • 'I can't take my holiday in the summer, so I'll take it sometime in the autumn.'
  • 'Our first child was born in 1996, so he'll be five years old in June.'
We also use in to describe how much time will pass before something happens or to talk about how long something took or takes. Consider the following:
  • 'Do you mind waiting? I shall be ready in about ten minutes.'
  • 'If you order it now, you'll receive it in about two weeks' time.'
  • 'I can run one hundred metres in 12.5 seconds!'
on with time phrases

We use on, Marta, to refer to particular days and dates, even repeated ones when plural forms are used. Consider the following:

  • 'Could we meet on Sunday morning?' 'No, not on Sunday. I go to church on Sundays.'
  • 'Why don't we have the meeting sometime in the afternoon on Thursday 5th April?'
  • 'It's my birthday on 22nd April, so I'll ring you on 23rd.'

Note that when we specify dates in writing, we will tend to write them in one of the following styles:

21 April 2001 (as part of letter heading)
on 29th December I'm leaving for Paris (within the body of the letter)

However, when we are speaking those dates, we will normally insert the definite article and the preposition of, as follows: I'm leaving for Paris on the twenty ninth of December. I'm leaving for Paris on December the twenty ninth.

zero preposition with time phrases

At/in/on are not normally used with time phrases starting with next, last, this, that, every, some, all. Consider the following:

  • 'Last year I made a cake for Jenny's birthday, but this year I'm going to buy one.'
  • 'Are you free this morning? If not, I'll see you next week.'
  • 'I'm at home all day tomorrow, so come round (at) any time.'
Finally, note that prepositions are often omitted from time questions starting with What...? or Which? Look at the following examples:
  • 'What time are you leaving?' 'At eight o' clock.'
  • 'Which days are you busy next week?' 'I'm busy on Wednesday and Friday, but I'm free on Thursday.'