Comparing DATEADD and DATEDIFF

The DATEADD function adds an interval to a date you specify. For example, if the due dates of all orders in the SalesOrderHeader table slipped 3 days, you could obtain the new dates with the following statement:

USE AdventureWorks;
GO
SELECT DATEADD(day, 3, DueDate)
FROM Sales.SalesOrderHeader;
GO

If the date parameter is a smalldatetime data type, the result is also a smalldatetime. You can use DATEADD to add seconds or milliseconds to a smalldatetime value, but doing this is meaningful only if the result date returned by DATEADD changes by at least 1 minute.

The DATEDIFF function calculates the period of time in dateparts between the second and first of two dates you specify. In other words, it finds an interval between two dates. The result is a signed integer value equal to date2 - date1 in date parts.

The following query uses the date November 30, 2001, and finds the number of days that elapsed between DueDate and that date:

USE AdventureWorks;
GO
SELECT DATEDIFF(day, DueDate, 'Nov 30 2001')
FROM Sales.SalesOrderHeader;
GO

For the rows in SalesOrderHeader that have having a DueDate of October 21, 2001, the result produced by the last query is 40. (There are 40 days between October 21 and November 30.) To calculate an interval in months, use the following query:

USE AdventureWorks;
GO
SELECT interval = DATEDIFF(month, DueDate, 'Nov 30 2001')
FROM Sales.SalesOrderHeader;
GO

The query produces a value of 1 for the rows with a DueDate in October and a value of 5 for the rows with a DueDate in June.

When the first date in the DATEDIFF function is later than the second date specified, the resulting value is negative.

If one or both of the date arguments is a smalldatetime value, they are converted to datetime values internally for the calculation. Seconds and milliseconds in smalldatetime values are automatically set to 0 for calculation.

SELECT DATEDIFF(year, '20051220', '20060101')
SELECT DATEDIFF(month, '20051220', '20060101')
SELECT DATEDIFF(day, '20051220', '20060101')

See Also

Other Resources

CAST and CONVERT (Transact-SQL)
DATENAME (Transact-SQL)
Date and Time Functions (Transact-SQL)
DATEPART (Transact-SQL)
DATEADD (Transact-SQL)
GETDATE (Transact-SQL)
DATEDIFF (Transact-SQL)
SET DATEFORMAT (Transact-SQL)

Help and Information

Getting SQL Server 2005 Assistance

Change History