DATEDIFF( date-part, date-expression-1, date-expression-2 )

date-part :
year 
| quarter 
| month 
| week 
| day 
| dayofyear 
| hour
| minute 
| second 
| millisecond
| microsecond

INT with year, quarter, month, week, day, and dayofyear. BIGINT with hour, minute, second, millisecond, and microsecond.

This function calculates the number of date parts between two specified dates. The result is a signed integer value equal to (date-expression-2 - date-expression-1), in date parts.

The DATEDIFF function results are truncated, not rounded, when the result is not an even multiple of the date part.

When you use day as the date part, the DATEDIFF function returns the number of midnights between the two times specified, including the second date but not the first.

When you use month as the date part, the DATEDIFF function returns the number of first-of-the-months between two dates, including the second date but not the first.

When you use week as the date part, the DATEDIFF function returns the number of Sundays between the two dates, including the second date but not the first.

The following statement returns 1.

SELECT DATEDIFF( hour, '4:00AM', '5:50AM' );

The following statement returns 102.

SELECT DATEDIFF( month, '1987/05/02', '1995/11/15' );

The following statement returns 0.

SELECT DATEDIFF( day, '00:00', '23:59' );

The following statement returns 4.

SELECT DATEDIFF( day,
   '1999/07/19 00:00',
   '1999/07/23 23:59' );

The following statement returns 0.

SELECT DATEDIFF( month, '1999/07/19', '1999/07/23' );

The following statement returns 1.

SELECT DATEDIFF( month, '1999/07/19', '1999/08/23' );