How to convert DATETIME to FILETIME value in T-SQL? - tsql

Question

I need to convert a SQL Server DATETIME value to FILETIME in a T-SQL SELECT statement (on SQL Server 2000). Is there a built-in function to do this? If not, can someone help me figure out how to implement this conversion routine as a UDF (or just plain Transact-SQL)? Here is what I know:
FILETIME is 64-bit value representing the number of 100-nanosecond intervals since
January 1, 1601 (UTC) (per MSDN: FILETIME Structure).
SQL Server base time starts on 1900-01-01 00:00:00 (per SELECT CAST(0 as DATETIME)).
I found several examples showing how to convert FILETIME values to T-SQL DATETIME (I'm not 100% sure they are accurate, though), but could not find anything about reverse conversion. Even the general idea (or algorithm) would help.

Answer 1

Okay, I think I was able to implement this myself. Here is the function:
IF EXISTS
(
SELECT 1
FROM sysobjects
WHERE id = OBJECT_ID('[dbo].[fnDateTimeToFileTime]')
AND type = 'FN'
)
BEGIN
DROP FUNCTION [dbo].[fnDateTimeToFileTime]
END
GO
-- Create function.
CREATE FUNCTION [dbo].[fnDateTimeToFileTime]
(
#DateTime AS DATETIME
)
RETURNS
BIGINT
BEGIN
IF #DateTime IS NULL
RETURN NULL
DECLARE #MsecBetween1601And1970 BIGINT
DECLARE #MsecBetween1970AndDate BIGINT
SET #MsecBetween1601And1970 = 11644473600000
SET #MsecBetween1970AndDate =
DATEDIFF(ss, CAST('1970-01-01 00:00:00' as DATETIME), #DateTime) *
CAST(1000 AS BIGINT)
RETURN (#MsecBetween1601And1970 + #MsecBetween1970AndDate) * CAST(10000 AS BIGINT)
END
GO
IF ##ERROR = 0
GRANT EXECUTE ON [dbo].[fnDateTimeToFileTime] TO Public
GO
It seems to be accurate up to 1 second, which is okay with me (I could not make it more accurate due to data overflow). I used the TimeAndDate web tool to calculate the durations between dates.
What do you think?

Answer 2

2 SQL Server time era starts on
1900-01-01 00:00:00 (per SELECT CAST(0
as DATETIME).
No, that is the base date, datetime starts at 1753
run this
select cast('17800122' as datetime)
output
1780-01-22 00:00:00.000
But this is still less than filetime so you need to add that...however remember the gregorian and Julian calendars (also the reason that datetime starts at 1753)

Answer 3

The accepted answer work well, but will crash for date above 19 January 2038. Either use
DATEDIFF_BIG instead of DATEDIFF if you are on SQL Server 2016 or above, or use the following correction
CREATE FUNCTION [dbo].[fnDateTimeToFileTime]
(
#DateTime AS DATETIME
)
RETURNS
BIGINT
BEGIN
IF #DateTime IS NULL
RETURN NULL
DECLARE #MsecBetween1601And1970 BIGINT
DECLARE #MsecBetween1970AndDate BIGINT
DECLARE #MaxNumberDayBeforeOverflowDateDiff int;
SET #MaxNumberDayBeforeOverflowDateDiff = 24855; --SELECT DATEDIFF(day, CAST('1970-01-01 00:00:00' as DATETIME), CAST('2038-01-19 00:00:00' as DATETIME))
DECLARE #nbMaxDaysBetween1970AndDate int;
SET #nbMaxDaysBetween1970AndDate = DATEDIFF(day, CAST('1970-01-01 00:00:00' as DATETIME), #DateTime) / #MaxNumberDayBeforeOverflowDateDiff;
DECLARE #moduloResteDay int
SET #moduloResteDay = DATEDIFF(day, CAST('1970-01-01 00:00:00' as DATETIME), #DateTime) % #MaxNumberDayBeforeOverflowDateDiff;
DECLARE #nbSecondBefore19700101And20380119 bigint = 2147472000;
SET #MsecBetween1601And1970 = 11644473600000;
DECLARE #DateTimeModulo datetime;
SET #DateTimeModulo = DATEADD(day, -#nbMaxDaysBetween1970AndDate * #MaxNumberDayBeforeOverflowDateDiff, #DateTime)
SET #MsecBetween1970AndDate = CAST(CAST(#nbMaxDaysBetween1970AndDate as bigint) * #nbSecondBefore19700101And20380119 +
DATEDIFF(ss, CAST('1970-01-01 00:00:00' as DATETIME), #DateTimeModulo) as bigint)*
CAST(1000 AS BIGINT)
RETURN (#MsecBetween1601And1970 + #MsecBetween1970AndDate) * CAST(10000 AS BIGINT)
END