Sql-server – more concise way to convert a UTC datetime into a local date only

query-refactorsql serversql-server-2005

I am trying to write a query that groups records based on the local date part only of a UTC datetime field.

For example, if my table contains 10/19/2012 2:00:00, then it should get grouped as 10/18/2012, since my local time is EST (-5h) and I'm only interested in the date portion of the field.

I know I can use DateAdd(day, DateDiff(day, 0, MyDate), 0) to get the date part only from the datetime field, and I can use DateAdd(minute, DateDiff(minute, GetUtcDate(), GetDate()), MyUtcDate) to convert a UTC datetime to a local date time.

But combining the two is seriously offending me.

Is there a better way than this to get just the Date part of a UTC DateTime field, converted to local time, in SQL Server 2005?

  SELECT DateAdd(day, DateDiff(day, 0, DateAdd(minute, DateDiff(minute, GetUtcDate(), GetDate()), MyUtcDate)), 0)
       , Count(*)
    FROM MyTable
GROUP BY DateAdd(day, DateDiff(day, 0, DateAdd(minute, DateDiff(minute, GetUtcDate(), GetDate()), MyUtcDate)), 0)

Best Answer

If you're not averse to having a function do the dirty work, this helps make the statement cleaner:

CREATE FUNCTION LocalDateFromUTCTime
(
    @UTCDateTime datetime
)
RETURNS datetime
BEGIN
    DECLARE @diff int;
    SET @diff = datediff(hh,GetUTCDate(), GetDate());
    RETURN DATEADD(day, DATEDIFF(day, 0, DATEADD(hh, @diff, @UTCDateTime)),0);
END

You could then do something like:

SELECT dbo.LocalDateFromUTCTime(MyUTCDate), COUNT(*)
FROM MyTable
GROUP BY dbo.LocalDateFromUTCTime(MyUTCDate);

This does, of course, make the statement non-SARGable.

If sargability is of primary concern due to the large number of records you may have, you could create a materialized view with an index on the calculated field.

EDIT:

Per @RichardThekiwi the SARGability of this particular statement is not affected by use of the function since the function is not part of either a JOIN or a WHERE clause. Where the function is used, it is ran after any indexes would have been used.

ALSO, not the above code will truncate any time portion of the date input to the function (this is by design). Therefore, any time zones that implement minutes such as the Prince Edward Island in Canada (UTC -4:30), India (UTC -4:30), and Kathmandu (UTC +5:45)

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