How do I stop SQL Server swapping the month and day? - sql

Question

I have the following query:
DECLARE #someDateTime DATETIME = '2019-12-01 00:00:00.000'; -- 1st December 2019
SELECT #someDateTime;
Result
2019-01-12 00:00:00.000
I expected the result to be 2019-12-01 00:00:00.000 (1st December 2019) - The month and the date swap for some unknown reason.
Until recently, I never had an issue with this format.
How do I enter the date in the format "YYYY-MM-DD HH:mm:ss.000", and have it maintain that format once assigned to a variable/displayed in a select?
What setting determines this format that may have changed?
Potentialy useful information
dbcc useroptions
Result:
Set Option | Value
---------- -----
...
language | British
dateformat | dmy
...
Things I tried:
Query 1
Chaning the date to something that would be invalid if swapped
DECLARE #someDateTime DATETIME = '2019-12-20 00:00:00.000';
SELECT #someDateTime;
Result:
Msg 242, Level 16, State 3, Line 4
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.
Query 2
Setting the variable after it's been declared
DECLARE #someDateTime DATETIME;
SET #someDateTime = '2019-12-01 00:00:00.000';
SELECT #someDateTime;
Result - UNDESIRED:
2019-01-12 00:00:00.000
Query 3
Inserting the variable into a table variable
DECLARE #someDateTime DATETIME = '2019-12-01 00:00:00.000';
DECLARE #someTable TABLE (someDateTimeColumn DATETIME);
INSERT #someTable VALUES (#someDateTime);
SELECT * FROM #someTable
Result - UNDESIRED:
someDateTimeColumn
------------------
2019-01-12 00:00:00.000
Query 4
Inserting the data directly into a table variable
DECLARE #someTable TABLE (someDateTimeColumn DATETIME);
INSERT #someTable VALUES ('2019-12-01 00:00:00.000');
SELECT * FROM #someTable
Result: UNDESIRED
someDateTimeColumn
------------------
2019-01-12 00:00:00.000
Query 5
Changing the format of the entered string
DECLARE #someDateTime DATETIME = '01/12/2019';
SELECT #someDateTime;
Result - DESIRED
2019-12-01 00:00:00.000
Query 6
Changing the format of the entered string
DECLARE #someDateTime DATETIME = '2019-12-01T00:00:00.00';
SELECT #someDateTime;
Result - DESIRED:
2019-12-01 00:00:00.000
Query 7
Changing the format of the entered string
DECLARE #someDateTime DATETIME = '2019-12-01';
SELECT #someDateTime;
Result - UNDESIRED:
2019-01-12 00:00:00.000
Query 8
Using SET DATEFORMAT
SET DATEFORMAT ymd
DECLARE #someDateTime DATETIME = '2019-12-01';
SELECT #someDateTime;
Result - DESIRED
2019-12-01 00:00:00.000

Answer 1

You may use the literal format YYYYMMDD which is always intepreted as year-month-day, regardless of the locale settings of your SQL Server:
DECLARE #someDateTime DATETIME = '20191201 00:00:00.000';

Answer 2

One solution is to use the full ISO8601 format, ie 2019-12-01T00:00:00.000, another to use the unseparated date format, ie 20191201 00:00:00.000. A better solution though would be to switch to datetime2.
datetime2 was introduced 15 (or was it 11) years ago to get rid of datetime's quirks, like the millisecond inaccuracy, arbitrary precision, weird arithmetic and ... parsing idiosyncrasies. For example, datetime2's parsing of the ISO8601 format isn't affected by DATEFORMAT :
SET DATEFORMAT ydm
DECLARE #someDateTime DATETIME2(0) = '2019-12-01 00:00:00.000'
select #someDateTime
------
2019-12-01 00:00:00
This secures your code from unfortunate server setting modifications

Answer 3

After reading this article from #SMor's comment, a comment my colleague made from a google search, and remembering something I did a few months ago, I think I've worked out what's happening and what's changed.
My db user was, by default, set to:
language | us_english
dateformat | mdy
So when reading the string '2019-12-01' SQL Server was expecting mm-dd-yyyy.
SQL server is clever though. It sees the first part 2019 and realises that it's actually the year, so it shifts the year to the end, then tries again.*
It now has 12-01-2019 which matches the format it's expecting.
When you open your account properties in SSMS, the language drop down defaults to the first language in the list (Arabic).
I happend to be in there a few months ago for something unrelated and decided, to make sure I didn't accidently set it to Arabic, I'd change that as well.
I chose British.
My db user is now set to:
language | British
dateformat | dmy
So when reading the string '2019-12-01' SQL Server is expecting dd-mm-yyyy.
Once it's shifted the year, it becomes 12-01-2019 which it interprets as dd-mm-yyyy thus leaving me with January 12th 2019 instead of December 1st 2019
Today has been a long day.
*Please note this is a rather simplified explanation of how I understand this issue. This may not be how it actually functions in reality but works to satisfy my curiosity on this issue.