in, on and at (time)

From: Oxford Practice Grammar : John Eastwood



in, on, at (time)

A. Saying when
Look at these examples:

It happened in 1990.

It happened on Friday.

It happened at two thirty.

Now compare:

in+year/month/season
in 1990
in September
in winter
in the 20th century

in + a week or more:
in the Eater holiday
in the summer term

in + part of the day:
in the morning
in the evenings

____________________________________

on + day/date:
on Wednesday
on 15th April
on that day

on + a single day:
on Easter Monday
on Christmas Day

on + day + part of day:
on Friday morning
on Tuesday evenings

___________________________________

at + clock time/meal time:
at three o'clock
at lunch (time)
at that time
at that moment

at + two or three days:
at Easter/Christmas
at the weekend
(USA: on the weekend)

___________________________________

in time or on time?

In time means "early enough" :

We got to the airport in time to have coffee before checking in.

We'll have to hurry if we want to be in time for the show.

I was about to close the door when just in time I remembered my key.

...

On time means "at the right time", 'on schedule'

The plane took off on time.
I hope the meeting starts on time.


Other meanings of in

We can use in for the time it takes to complete something:

I did the crossword in five minutes.

Could you walk thirty minutes in a day?

***

We can also use in for a future time measured from the present:

Your photos will be ready in an hour. (=an hour from now)

The building will open in six weeks. (OR in six weeks' time).