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AT TIME ZONE is great because it makes it easy to perform daylight saving time and time zone conversions in our queries.

However, when using AT TIME ZONE for these types of conversions there are a couple key points to remember to ensure you are getting the correct results. Let's look at an example.

Thanks to reader Jeff Konicky for the inspiration for this post and allowing me to share it.

DST Conversions

This year, Daylight Savings Time kicks in at 2019-03-10 02:00:00 in Eastern Standard Time, meaning that the time zone switches from being 5 hours behind UTC to only 4 hours behind UTC.

If we have two datetime2s with UTC data, one right before DST kicks in and one right after, we should be able to use AT TIME ZONE to convert these UTC times to Eastern Standard/Daylight Time no problem:

DECLARE 
    @PreDST datetime2 = '2019-03-10 06:59:00',
    @PostDST datetime2 = '2019-03-19 07:00:00';

SELECT 
    @PreDST AT TIME ZONE 'Eastern Standard Time' AS [PreDST should be -05:00],
    @PostDST AT TIME ZONE 'Eastern Standard Time' AS [PostDST should be -04:00];

While that sounds great, the results above show it clearly didn't work - both of those datetime2s are showing as already being in daylight saving time (offset of -04:00).

Why did this happen?

SQL Server Doesn't Know Your Data Is In UTC

The fallacy above is that I said our two datetime2's are in UTC, but SQL Server doesn't actually know this. The datetime2 (and datetime) datatype doesn't allow for time zone offsets so SQL Server really doesn't know what time zone the data is in.

Using AT TIME ZONE on a datetime2 without offset information causes SQL Server to "...[assume] that [the datetime] is in the target time zone". That explains why the two datetime2s above, intended to be in UTC, are actually seen as Eastern Daylight Time by SQL Server.

Specifying the Time Zone Offset

So how do we tell SQL Server that our data is in UTC so AT TIME ZONE functions like we want?

One option is to use the assumption of AT TIME ZONE above in our favor, using it to first convert the datetime2s to UTC and then to EST:

SELECT 
    @PreDST AT TIME ZONE 'UTC' AT TIME ZONE 'Eastern Standard Time' AS [PreDST should be -05:00],
    @PostDST AT TIME ZONE 'UTC' AT TIME ZONE 'Eastern Standard Time' AS [PostDST should be -04:00];

Now we have the correct -05:00 and -04:00 offsets applied.

A cleaner solution would be to encode the original data by using the datetimeoffset datatype, which will indicate which time zone the datetimes are stored in:

DECLARE 
    @PreDST datetimeoffset = '2019-03-10 06:59:00+00:00',
    @PostDST datetimeoffset = '2019-03-19 07:00:00+00:00';

SELECT 
    @PreDST AT TIME ZONE 'Eastern Standard Time' AS [PreDST should be -05:00],
    @PostDST AT TIME ZONE 'Eastern Standard Time' AS [PostDST should be -04:00];

Don't Assume

Whenever working with datetime2 (or datetime) in SQL Server, you can't assume SQL Server knows the time zone of your data. Your business logic may indicate that you store dates in UTC, but SQL Server has no way of knowing that without your code explicitly stating it using something like AT TIME ZONE 'UTC' or storing your data in the datetimeoffset datatype.

I do my best work in the mornings.  Evenings are pretty good too once I get a second wind.

Late afternoon are my nemesis for getting any serious technical or creative work done.  Usually I reserve that time for responding to emails, writing documentation, and brewing coffee.

Some afternoons I can't help myself though and end up getting myself into trouble.

What is THAT!?

It all started when I was troubleshooting an existing query that was having issues.  During the process of trying to understand what the query was doing, I happened to look at the execution plan:

CREATE TABLE ##DatetimeTest ( SomeField varchar(50) NULL, CreateDatetime datetime);
INSERT INTO ##DatetimeTest VALUES ('asdf',GETDATE());

DECLARE @Today datetime = GETDATE();
SELECT DISTINCT
    *
FROM 
    ##DatetimeTest
WHERE
    CreateDatetime >= @Today-30;

When I hovered over the Table Scan, the Predicate section caught my eye.  Specifically, I wondered "Why is 1900-01-31 showing up? I don't have that anywhere in my query!"

(fun side story: the real query I was working on was dealing with user-defined datatypes, something I don't have experience with, so I thought those had something to do with the problems I was experiencing.  I latched onto this 1900 date as the potential cause but it ended up being a red herring!)

Help!

Like I mentioned, late afternoons are not when I do my best work.  I couldn't figure out why SQL Server was converting my -30 to January 31, 1900.

Intrigued and having no clue what was going on, I decided to post the question with the #sqlhelp hashtag on Twitter.  Fortunately for me, Aaron Bertrand, Jason Leiser, and Thomas LaRock all came to the rescue with ideas and answers - thanks guys!

Implicit Conversion

In hindsight, the answer is obvious: the -30 implicitly converts to a datetime (the return type of my @Today variable), in this case 30 days after the start of the minimum datetime value, 1900-01-01.

This makes perfect sense: SQL Server needs to do some math and in order to do so it first needs to make sure both datatypes in the equation match.  Since int readily converts to datetime but not the other way around, SQL Server was just doing its job.

Future Problems

As I mentioned earlier, this int to datetime conversion wasn't the actual issue with my query - in my drowsy state I mistook it as being the source of my problem.

And while it wasn't a problem this time, it can become a problem in the future.

Aaron has an excellent article on the problems with shorthand date math, but the most relevant future issue with my query is: what if someone in the future decides to update all datetimes to datetime2s (datetime2 being Microsoft's recommended datatype for new work)?

If we update to a datetime2s and run the query again:

ALTER TABLE ##DatetimeTest
ALTER COLUMN CreateDatetime datetime2;

DECLARE @Today datetime2 = GETDATE();
SELECT DISTINCT
    *
FROM 
    ##DatetimeTest
WHERE
    CreateDatetime >= @Today-30;

AHHH!  While SQL Server had no problem converting our previous code between datetime and int, it's not so happy about converting datetime2.

Morals

In the end, the above scenario had nothing to do with the actual problem I had on hand (which had to do with some operator precedence confusion).

To avoid future confusion and problems it's still better to refactor the code to be explicit with what you want to do by using the DATEADD() function:

DECLARE @Today datetime = GETDATE();
SELECT DISTINCT
    *
FROM 
    ##DatetimeTest
WHERE
    CreateDatetime >= DATEADD(day, -30, @Today);