Prepositions of Time
There are three prepositions of time:
‘At’ is used for precise times:
At ten o’clock.
At the moment/present/same time.
Note: At night is a standard expression that is an exception in this case.
‘In’ is used for months, years, decades, centuries, and long periods of time:
In the 70s.
In this century.
In the Dark Ages.
Note: In the morning(s)/afternoon(s)/evening(s) are standard expressions that are exceptions in this case.
‘On’ is used for days and dates:
On the 10th of January
Note: ‘on’ is also used when you specify the day followed by the part of the day. For example on Wednesday mornings.