Date and time abbreviations

My brain does not function well after 1 am.

‘Am’ is an abbreviation and stands for ‘ante meridian’, which means the period of time between midnight and noon. ‘Pm’ stands for ‘post meridian’, which means the period of time between noon and midnight. Incidentally, it’s generally agreed that midnight itself counts as 12 am and that noon itself counts as 12 pm.

I’ve seen both lowercase and uppercase letters used for ‘am’ and ‘pm’ - you can probably use either. However, when you’re talking about the date in terms of years, you have to use capital letters:

BC - before Christ
AD - anno domini, which means literally ‘in the year of the Lord’.

When you use ‘BC’ you should put it after the number indicating the year. However, when you use ‘AD’ you need to put it before the number indicating the year. You can remember this by how it is opposite to the ‘before’ in ‘before Christ’:

BC - before Christ, so is put after the year number
AD - opposite of ‘BC’, so comes before the year number.

Examples of usage:

Julius Caesar was born in 100 BC.

Atilla the Hun was born in about AD 406.

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