It's something that all of a sudden when you think you need it. You cant think of how to do it.

So here is a quick function that turns a unix timestamp into a datetime

CREATE FUNCTION [dbo].[UNIXToDateTime] (@timestamp int)
	RETURNS datetime
AS
BEGIN
	DECLARE @ret datetime
	
	SELECT @ret = DATEADD(second, @timestamp, '1970/01/01 00:00:00')
	
	RETURN @ret
END
GO

An example on how to use it

SELECT dbo.UNIXToDateTime(dbo.DateTimeToUNIX(GETDATE()))

You may also want to see the convert to unix timstamp function