What is the function used to calculate number of days , months or years between two given dates ?

if you want to get the days:

you just simply subtract the newest date from the older date.

if you want the difference in months after days:

you can use the DateDif(StartDate, EndDate,"md")

done.

=DATEDIF(STARTDDATE,ENDDATE,"Y")

Use the DATEDIF function when you want to calculate the difference between two dates. First put a start date in a cell, and an end date in another. 

DATEDIF function will calculate difference between two dates and depending on which unit you choose at the end of the formula (years, months, days) it will calculate the one you specified. 

The function used to calculate number of days, months or years between two given dates is DATEDIF