DATEDIFF(day, '9-8-1989', '10-17-1991')

1>
2> SELECT DATEDIFF(day, '9-8-1989', '10-17-1991')
3> GO

-----------
        769

(1 rows affected)
1>

1.	DATEDIFF: return the difference between two dates
2.	select DATEDIFF(second, '06:46:45', '11:35:00')
3.	select DATEDIFF(quarter, '2001-12-01', '2002-09-30')
4.	Find the number of days between 24th March 2001 and 24th March 1964
5.	DATEDIFF(yyyy, "24 March 2001","24 March 1964")
6.	DATEDIFF(mi, "24 March 2001","24 March 1964")
7.	DATEDIFF(wk, "24 March 2001","24 March 1964")
8.	DATEDIFF(m, "24 March 2001","24 March 1964")
9.	DATEDIFF(d, @thisdate, @nextmonth) 'Days between'
10.	DATEDIFF(month, '9-8-1989', '10-17-1991')
11.	Difference in weeks between 7-2-1996 and 8-4-1997
12.	DATEDIFF(ss, @MyBirthDate, GETDATE())
13.	select DATEDIFF(hour, '06:46:45', '11:35:00')
14.	select DATEDIFF(minute, '06:46:45', '11:35:00')
15.	Difference between the current time and UTC
16.	DATEDIFF: date difference
17.	Combine DateADD and DateDIFF to get the last date of a month
18.	Passing a column name to the DATEDIFF() value argument

DATEDIFF(day, '9-8-1989', '10-17-1991') : DATEDIFF « Date Timezone « SQL Server / T-SQL