DATEDIFF
Returns the number of date and time boundaries crossed between two specified dates.
For example, you could use this function to find the number of days in a certain interval of dates.
Note:  This function 
 is equivalent to DATEDIFF in T-SQL.
      Return Value
Integer
Syntax
DATEDIFF( datePart, startDate, endDate )
where:
- 
               datePart specifies the part of the 
	 dates on which to calculate the difference. Possible values for this 
	 argument include the following: 
	
- year, yy, yyyy
 - quarter, q, qq
 - month, m, mm
 - week, wk, ww
 - day, d, dd
 - weekday, dw
 - dayofyear, dy, y
 - hour, hh
 - minute, mm, n
 - second, ss, s
 - millisecond, ms
 
 - startDate is the beginning date for the calculation.
 - endDate is the ending date for the calculation.
 
The startDate value is subtracted from the endDate.
Note:  If the startDate 
 is later than the endDate, then the return 
 value is a negative number. If the result is out of range for integer 
 values, the system returns an error.