The DATETIME_DIFF() function will allow you to calculate the difference between two different datetimes in specified units. Each specifier has a corresponding abbreviation which will work the same as the longform version of the specifier, e.g. DATETIME_DIFF(Date, Date2, 'y') will have the same output as DATETIME_DIFF(Date, Date2, 'years'). If no unit is specified, or the specifier is not recognized by the function, then the output will default to seconds.
The following is a table of the supported time units for DATETIME_DIFF(), along with the equivalent abbreviated specifiers for each:
If you're looking to use workdays as a unit for DATETIME_DIFF(), you may want to use the formula WORKDAY_DIFF() instead.