To determine if a time is AM or PM, use the following formula:

Here, A1 is the cell containing the time you want to check. The TIME function is used to specify the time 12:00 PM, and the IF function checks if the time in A1 is greater than or equal to 12:00 PM. If it is, the formula returns "PM", otherwise it returns "AM".

Make sure that the cell containing the time is formatted as a time value, otherwise the formula may not work correctly. To format a cell as a time value, select the cell and go to the Home tab, click on the Number Format dropdown, and select "Time" from the list.