There are 21 date and time functions varying in usefulness, power and potential complexity.

Date and Time Creation Functions

Function Usefulness Power Complexity
DATE
DATEVALUE
TIME
TIMEVALUE

Current Date and Time Functions

Function Usefulness Power Complexity
NOW
TODAY

Date and Time Math Functions

Date Part Functions

Function Usefulness Power Complexity
DAY
YEAR

Time Part Functions

Function Usefulness Power Complexity
HOUR
MINUTE
SECOND
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Using a Date and Time function:
Using a Date and Time Excel function

Summary

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".