This often quoted sentence reads like this:

• The First World War ended at 11 a.m. on the eleventh day of the eleventh month in 1918.

at / around / about / by

We use at when we are discussing precise times, weekends and public holidays:

• What time does your train get in? ~ It gets in at five twenty five.
• Let's meet at Waterloo Station at the end of platform one at five thirty.
• What are you doing at the weekend?
  ~ I'm going to see my parents at Easter, but I've got no plans for next weekend.

We use around or about when we want to indicate approximate times:

• When does his train get in?
  ~ Around / About ten o' clock usually, but you can never be sure these days!
• What time should I come? ~ Come about / around eight.

We use by to indicate at or before, not later than. It can also suggest progression up to a certain time:

• You must be here by / not later than 10.45 if we want to catch the eleven o' clock train.
• By the time I arrived home, both children were in bed and asleep.
  

We use on when we are discussing particular days and in the expression on weekdays:

• My daughter's birthday is on 29th February.
• She was born on 29th February 2000, so she won't be one till 29th February 2004!
• My aerobic classes are on Tuesday evenings.
• Peter's tennis lessons are on Thursdays at lunchtime.
• On weekdays I get up at seven, but at weekends I lie in until nine.

Note that we can write dates as 29th February 2002 or 29 February 2002. In formal letters it is usually the latter. When we are speaking though we usually say: the 29th of February 2002.

till / until

Both till and until mean up to the time indicated or up to the time when. Note that they can both be used as conjunctions introducing clauses as well as prepositions introducing time phrases:

• Can you work today till seven?
  ~ No, I'm sorry. I have to leave at five.
• Until Tom met Jane, he had always visited his mum at weekends.
• Can you stay over till Monday morning?
  ~ OK. But I'll have to be gone by six thirty.

in

We use in when we are discussing parts of the day or longer periods:

• I'm happy to work in the morning, but I always have a snooze in the afternoon.
• I don't mind working in the evening, but I hate to get up in the night.
• These fox cubs were born in the spring. ~ In April or May? ~ I'm not sure. In April, I think.
• They'll be ready to leave home and fend for themselves in about three weeks from now.

during

We can use during when we are discussing something happening between the beginning and end of an activity:

• In / During summer, I try to have a nap at some point during the afternoon.
• But I never bother with siestas during autumn, winter or spring.
• Could you please try not to interrupt me during this meeting?
• There's an answer phone message from John. He must have phoned during lunch.

Note that there are some time expressions, typically involving all, some, any, each, every, this, that, last, next, where no preposition is needed:

• The last time we met was at Sheila's birthday. Try not to leave it so long next time.
• Where were you last Tuesday?
  ~ Sorry, I was out, but I'll be in all this week. Feel free to call any evening.
• How often do you text message your sister?
  ~ Every day. Sometimes twice a day.
  

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