Convert .NET Ticks to SQL Server DateTime

I'm not sure how accurate this will be with the seconds, but you could try something like:

Declare @TickValue bigint
Declare @Days float

Set @TickValue = 634024345696365272 
Select @Days = @TickValue * POWER(10.00000000000,-7) / 60 / 60 / 24

Select DATEADD(d, Cast(@Days As int), Cast('0001-01-01' As DATE)) 
    + Cast( (@Days - FLOOR(@Days)) As DateTime)

Actually another way that would work in SQL 2005 is to note that the the number of ticks from 0001-01-01 to 1900-01-01 is 599266080000000000. With that you could do:

Declare @TickOf19000101 bigint
Declare @TickValue bigint
Declare @Minutes float

Set @TickOf19000101  = 599266080000000000
Set @TickValue = DATEDIFF(mi, 0 ,CURRENT_TIMESTAMP) * Cast(60 As BigInt) 
                   * POWER(10.00000000000,7) + @TickOf19000101

Select @TickValue
Select @Minutes = (@TickValue - @TickOf19000101) * POWER(10.00000000000,-7) / 60

Select @Minutes
Select DATEADD(MI, @Minutes, '1900-01-01')

A TimeSpan is not a date, and saving it as such may cause confusion in the future.

Is there a reason you can't simply save the ticks to an integer field and not change its meaning?

I don't really know SQL Server, but today a colleague of mine had the same problem and I think I've found a solution like this:

CAST(([ticks] - 599266080000000000) / 10000000 / 24 / 60 / 60 AS datetime)

where 599266080000000000 is the ticks value for 01/01/1900 00:00:00.

You can use this function taken from Pavel Gatilov's blog to convert a 64-bit integer to a datetime value with millisecond precision in server local time:

CREATE FUNCTION NetFxUtcTicksToDateTime
(
   @Ticks bigint
)
RETURNS datetime
AS
BEGIN

-- First, we will convert the ticks into a datetime value with UTC time
DECLARE @BaseDate datetime;
SET @BaseDate = '01/01/1900';

DECLARE @NetFxTicksFromBaseDate bigint;
SET @NetFxTicksFromBaseDate = @Ticks - 599266080000000000;
-- The numeric constant is the number of .Net Ticks between the System.DateTime.MinValue (01/01/0001) and the SQL Server datetime base date (01/01/1900)

DECLARE @DaysFromBaseDate int;
SET @DaysFromBaseDate = @NetFxTicksFromBaseDate / 864000000000;
-- The numeric constant is the number of .Net Ticks in a single day.

DECLARE @TimeOfDayInTicks bigint;
SET @TimeOfDayInTicks = @NetFxTicksFromBaseDate - @DaysFromBaseDate * 864000000000;

DECLARE @TimeOfDayInMilliseconds int;
SET @TimeOfDayInMilliseconds = @TimeOfDayInTicks / 10000;
-- A Tick equals to 100 nanoseconds which is 0.0001 milliseconds

DECLARE @UtcDate datetime;
SET @UtcDate = DATEADD(ms, @TimeOfDayInMilliseconds, DATEADD(d, @DaysFromBaseDate, @BaseDate));
-- The @UtcDate is already useful. If you need the time in UTC, just return this value.

-- Now, some magic to get the local time
RETURN @UtcDate + GETDATE() - GETUTCDATE();
END
GO

Alternative code suitable for inline usage:

DECLARE @Ticks bigint
set @Ticks = 634899090000000000
select DATEADD(ms, ((@Ticks - 599266080000000000) - 
   FLOOR((@Ticks - 599266080000000000) / 864000000000) * 864000000000) / 10000,
   DATEADD(d, (@Ticks - 599266080000000000) / 864000000000, '01/01/1900')) +
   GETDATE() - GETUTCDATE()