DATEDIFF() returns a value in datepart format, specifying the difference between two specified dates.
Syntax : DATEDIFF ( datepart , startdate , enddate )
DATEDIFF(yy, OrderDate, GETDATE()) YearsPassedSinceOrder
returns the no. of years passed since the order was placed.
Similarly to find number of months, use 'm' as the datepart or 'd' to determine no. of days.
You can find other values for datepart over here