Calculates the days, weeks, months between two dates.
Notes on use:
It is recommended you use the latest datetime value for the DATETIME_2. If you enter the earliest date as the DATETIME_2 value, the DATEDIFF function will return a negative number.
DATEDIFF always rounds the result down to the nearest whole number. For example, if the difference between two dates is 3 years and 11 months, the DATEDIFF function returns the difference as 3 years.