How to update rows with a random date

I have a simple SQL table which has a DateTime column. I would like to update all the rows (>100000 rows) with a random date. Is there a simple way to do this a SQL Query?

Use this to generate a smalldatetime between 01 Jan 1900 and 06 Jun 2079 (not checked, SQL not installed)

DATEADD(day, (ABS(CHECKSUM(NEWID())) % 65530), 0)

NEWID is better then trying to use RAND: RAND does not generate different values row in a single SELECT or UPDATE (well it didn't in SQL 2000, in case behaviour has changed).

Edit: like this

UPDATE
  table
SET
  datetimecol = DATEADD(day, (ABS(CHECKSUM(NEWID())) % 65530), 0)

Edit: changed 65535 to 65530 and added ABS to avoid overflow at upper limit of range

I will complement the answers below,

SELECT DATEADD(DAY, ABS(CHECKSUM(NEWID()) % 3650), '2000-01-01')
FROM your_table

This generates dates starting from 2000-01-01, and you can change the amount of days in the modulus value, I put 3650 (about 10 years), this approach doesn't overflow.

If you want to update, then

UPDATE your_table
SET your_date_field = DATEADD(DAY, ABS(CHECKSUM(NEWID()) % 3650), '2000-01-01')
WHERE your_conditions

This question seems quite old but my answer might be usefull to others.

      Update table
      SET Time= DateAdd(d, ROUND(DateDiff(d, '2010-01-01', '2013-12-31') * RAND(CHECKSUM(NEWID())), 0),
      DATEADD(second,CHECKSUM(NEWID())%48000, '2010-01-01'))

This generates a random datetime between a given range.

I adapted Jhonny's answer above to get dates from 10 years in the past:

SELECT dateadd(day, (abs(CHECKSUM(newid())) % 3650) * -1, getdate())

Note that this is SQLServer only.

I used this to set a date of birth between 1940 and 1985 for all of my test data

SET [Birth Date] = DATEADD(day, (ABS(CHECKSUM(NEWID())) % 16250), '1940-1-1 00:00:00.001')

The following code will fill the StartDate column of the FiscalYear table with random dates between two given dates:

-- First, let's declare the date range.
DECLARE @date_from DATETIME;
DECLARE @date_to DATETIME;
 
-- Set the start and date dates. In this case, we are using
-- the month of october, 2006.
SET @date_from = '1985-10-14';
SET @date_to = '2009-04-27';

UPDATE FiscalYear SET StartDate =  
(
	-- Remember, we want to add a random number to the
	-- start date. In SQL we can add days (as integers)
	-- to a date to increase the actually date/time
	-- object value.
	@date_from +
	(
		-- This will force our random number to be >= 0.
		ABS
		(
			-- This will give us a HUGE random number that
			-- might be negative or positive.
			CAST(CAST(NewID() AS BINARY(8)) AS INT)
		)

		-- Our random number might be HUGE. We can't have
		-- exceed the date range that we are given.
		-- Therefore, we have to take the modulus of the
		-- date range difference. This will give us between
		-- zero and one less than the date range.
		%

		-- To get the number of days in the date range, we
		-- can simply substrate the start date from the
		-- end date. At this point though, we have to cast
		-- to INT as SQL will not make any automatic
		-- conversions for us.
		CAST((@date_to - @date_from) AS INT)
	)
)

I was looking for a question similar to this that also generated a random time and I found this script. Thought it might be useful here:

DECLARE @DateFrom DATETime = '2001-01-01'
DECLARE @DateTo DATeTime = '2013-11-30'
DECLARE @DaysRandom Int= 0
DECLARE @MillisRandom Int=0

--get random number of days

select @DaysRandom= DATEDIFF(day,@DateFrom,@DateTo)
SELECT @DaysRandom = ROUND(((@DaysRandom -1) * RAND()), 0)
 
--get random millis
SELECT @MillisRandom = ROUND(((99999999) * RAND()), 0)
 
SELECT @DateTo = DATEADD(day, @DaysRandom, @DateFrom)
SELECT @DateTo = DATEADD(MILLISECOND, @MillisRandom, @DateTo)
SELECT @DateTo

I got it from here: http://crodrigues.com/sql-server-generate-random-datetime-within-a-range/

Using the code below you can get a random integer between @Min (1) and @Max (365), then using the dateadd funection you can create random dates in the last year.

CREATE VIEW vRandNumber
AS
SELECT RAND() as RandNumber
GO

CREATE FUNCTION RandNumber(@Min int, @Max int)
RETURNS int
AS
 BEGIN
 RETURN round(@Min + (select RandNumber from vRandNumber) * (@Max-@Min),0)
 END
GO

Update table1
set theDate = dateadd(d,0-dbo.RandNumber(1,365),getdate())

you can try getting a random number (positive or negative) then adding that number to a date (possibly system date).

For example (I don't have access to sqlserver right now so I could not verify syntax)

DATEADD(day, DATEDIFF(day, 0, GETDATE()) - 1 - FLOOR(RAND(CAST(NEWID() AS binary(4))) * 365.25 * 90), 0)

I combined several answers for myself, I think it it work for you. It took 40 seconds for me to execute this for 140k rows. i5, 1333MHZ, standart laptop hdd

 DECLARE @rank INT = 0;

WHILE @rank < yourmaxrow --(you can use Select count (*) from your table name as well)
BEGIN
   DECLARE @FromDate DATETIME = DATEADD(DAY, -720, GETDATE()) -- 2 years back
   DECLARE @ToDate   DATETIME = DATEADD(DAY, -1, GETDATE()) -- until yesterday

   DECLARE @Seconds INT = DATEDIFF(SECOND, @FromDate, @ToDate)
   DECLARE @Random INT = ROUND(((@Seconds-1) * RAND()), 0)
   DECLARE @Milliseconds INT = ROUND((999 * RAND()), 0)

update yourtablename
Set yourdatetiemcolumnname = DATEADD(MILLISECOND, @Milliseconds, DATEADD(SECOND, @Random, @FromDate))
WHERE Id = @rank
   SET @rank = @rank + 1;       
END;