SQL Server Date and Time Format using strings - sql-server

Question

I was always under the impression given a datetime data type as 2019-07-06 was always 6th July 2019, regardless of locale and regional settings, ie this is a universal datetime format.
However, I've ran into a situation where I am trying to query some data from our database, and the string 2019-07-06 does not return the expected results. However 2019-06-07 will.
I have always had the understanding that YYYY-MM-DD format was based on ISO-8601.
Given this was on Friday 7th June where I was inserting the data, and today I am querying that data, I see:
Using the query
select
b.ID
, b.UserId
, b.EntryDate
, day(b.EntryDate) as 'day'
, month(b.EntryDate) as 'month'
from [dbo].[SomeTable] b
where b.UserId = 236328
But when I change the query:
select
b.ID
, b.UserId
, b.EntryDate
, day(b.EntryDate) as 'day'
, month(b.EntryDate) as 'month'
from [dbo].[SomeTable] b
where b.UserId = 236328
and b.EntryDate > '2019-06-07'
This does not yield any results. However changing EntryDate > '2019-07-06' will return my data.
NOTE: The query is entered using SSMS v18 on my local machine.
Server Configuration
The current verison of SQL Server is Microsoft SQL Server 2012 (SP4-GDR) (KB4057116) - 11.0.7462.6 (X64) Jan 5 2018 22:11:56 Copyright (c) Microsoft Corporation Developer Edition (64-bit) on Windows NT 6.3 <X64> (Build 9600: )
The sql server is configured to use:
|------------------|------------------------------|
| Setting | Value |
|------------------|------------------------------|
| Language | English (United States) |
| Default Language | British English |
| Server Collation | SQL_Latin1_General_CP1_CI_AS |
Database Configuration
The database is configured to use:
|---------------------|-----------------------|
| Setting | Value |
|---------------------|-----------------------|
| Collation | Latin1_General_CI_AS |
| Compatability Level | SQL Server 2012 (110) |
| Default Language | British English |
Question is, what should be the expected behaviour of date time queries with strings with format YYYY-MM-dd? I cannot recall this behaviour I am seeing in the last 20 years.
Update
For the record, if I enter the datetime in the query as 2019-06-07T00:00:00 then this DOES return the expected data.

Answer 1

With the DATETIME type, the literal 2019-06-07 could be 7th June, or 6th July, depending on the current DATEFORMAT settings:
SET DATEFORMAT DMY;
SELECT FORMAT(CONVERT(DATETIME, '2019-06-07'), 'dd MMM yy');
--Returns "06 Jul 19"
SET DATEFORMAT MDY;
SELECT FORMAT(CONVERT(DATETIME, '2019-06-07'), 'dd MMM yy');
-- Returns "07 Jun 19"
You should not rely on your default server settings for this, the best way to avoid this ambiguity is to use the format yyyyMMdd, this is always interpreted the same way regardless of settings:
SET DATEFORMAT DMY;
SELECT FORMAT(CONVERT(DATETIME, '20190607'), 'dd MMM yy');
--Returns "07 Jun 19"
SET DATEFORMAT MDY;
SELECT FORMAT(CONVERT(DATETIME, '20190607'), 'dd MMM yy');
-- Returns "07 Jun 19"
Alternatively, do an explicit conversion, and use the style parameter to ensure consistent conversion:
SET DATEFORMAT DMY;
SELECT FORMAT(CONVERT(DATETIME, '2019-06-07', 101), 'dd MMM yy');
-- Returns "07 Jun 19"
SET DATEFORMAT MDY;
SELECT FORMAT(CONVERT(DATETIME, '2019-06-07', 101), 'dd MMM yy');
-- Returns "07 Jun 19"
As an aside there is no ambiguity with this format when working with DATE or DATETIME2, but this ambiguity has always existed for DATETIME and SMALLDATETIME. Aaron Bertrand blogged about this in his Bad habits to kick series: mis-handling date / range queries