There are 21 date and time functions varying in usefulness, power and potential complexity.
Date and Time Math Functions
Using a Date and Time function:
Date and time functions can vary in usefullness, but key to understanding these functions is understanding how Excel associates a serial number value to each day. Excel's date and time functions can be used in many ways. Dates can be added and subtracted to determine a span of time and you can extract a portion of a date, such as the day of month from a date or the hour portion of the time and so on. This is often referred to as doing "date math".