How can I convert bigint (UNIX timestamp) to datetime in SQL Server?

Like this

add the Unix (epoch) datetime to the base date in seconds

this will get it for now (2010-05-25 07:56:23.000)

 SELECT dateadd(s,1274756183,'19700101 05:00:00:000')

If you want to go reverse, take a look at this http://wiki.lessthandot.com/index.php/Epoch_Date

try:

CREATE FUNCTION dbo.fn_ConvertToDateTime (@Datetime BIGINT)
RETURNS DATETIME
AS
BEGIN
    DECLARE @LocalTimeOffset BIGINT
           ,@AdjustedLocalDatetime BIGINT;
    SET @LocalTimeOffset = DATEDIFF(second,GETDATE(),GETUTCDATE())
    SET @AdjustedLocalDatetime = @Datetime - @LocalTimeOffset
    RETURN (SELECT DATEADD(second,@AdjustedLocalDatetime, CAST('1970-01-01 00:00:00' AS datetime)))
END;
GO

This worked for me:

Select
    dateadd(S, [unixtime], '1970-01-01')
From [Table]

In case any one wonders why 1970-01-01, This is called Epoch time.

Below is a quote from Wikipedia:

The number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970,[1][note 1] not counting leap seconds.

The Year 2038 Problem

Furthermore the DataAdd function takes an int for the seconds to add. So if you are trying to add more than 2147483647 seconds you'll get an arithmetic overflow error. To resolve this issue you can break the addition into two calls to DateAdd one for the years and one for the remaining seconds.

Declare @t as bigint = 4147483645

Select (@t / @oneyear) -- Years to add
Select (@t % @oneyear) -- Remaining seconds to add

-- Get Date given a timestamp @t
Declare @oneyear as int = 31622400
Select DateAdd(SECOND, @t % @oneyear, DateAdd(YEAR, @t / @oneyear, '1970-01-01'))

This will let you convert a timestamp that represents a year greater than 2038.

If anyone getting below error:

Arithmetic overflow error converting expression to data type int

due to unix timestamp is in bigint (instead of int), you can use this:

SELECT DATEADD(S, CONVERT(int,LEFT(1462924862735870900, 10)), '1970-01-01')
FROM TABLE

Replace the hardcoded timestamp for your actual column with unix-timestamp

Source: MSSQL bigint Unix Timestamp to Datetime with milliseconds