DATEDIF(start_date,end_date,interval)
start_date: starting date serial value
end_date: ending date serial value
interval: counting unit
DATEDIF returns the distance from start_date to end_date according to the unit specified by interval.
If interval is "y", "m", or "d" then the distance is measured in complete years, months, or days respectively. If interval is "ym" or "yd" then the distance is measured in complete months or days, respectively, but excluding any difference in years. If interval is "md" then the distance is measured in complete days but excluding any difference in months.
This function is Excel compatible.