-- How most date/time formats are unsafe
-- Let's check the language of my session:

SET LANGUAGE us_english;
PRINT @@LANGUAGE;

-- Then we can see how  [`us_english`]  interprets various settings. First, April 12th / 20th:

SELECT [April_12_1] = CONVERT(datetime, '04/12/2020'), -- mm/dd/yyyy
       [April_12_2] = CONVERT(datetime, '2020-04-12'), -- yyyy-mm-dd (ISO 8601)
       [April_12_3] = CONVERT(datetime, '20200412'),   -- yyyymmdd

       [April_20_1] = CONVERT(datetime, '04/20/2020'), 
       [April_20_2] = CONVERT(datetime, '2020-04-20'), 
       [April_20_3] = CONVERT(datetime, '20200420');   

-- And formats that include time:

SELECT [April_12_a] = CONVERT(datetime, '2020-04-12 04:00:00'), -- yyyy-mm-dd hh:mm:ss[.nnn]
       [April_12_b] = CONVERT(datetime, '2020-04-12T04:00:00'), -- yyyy-mm-ddThh:mm:ss[.nnn]
       [April_12_c] = CONVERT(datetime, '20200412 04:00:00'),   -- yyyymmdd hh:mm:ss[.nnn]

       [April_20_a] = CONVERT(datetime, '2020-04-20 04:00:00'),
       [April_20_b] = CONVERT(datetime, '2020-04-20T04:00:00'), 
       [April_20_c] = CONVERT(datetime, '20200420 04:00:00');

-- Then what happens if a user has different language settings?

SET LANGUAGE français;

SELECT [April_12_1] = CONVERT(datetime, '04/12/2020'), -- December 4th!
       [April_12_2] = CONVERT(datetime, '2020-04-12'), -- December 4th!
       [April_12_3] = CONVERT(datetime, '20200412');   -- April 12th

-- That's just wrong data, and you have no idea.

-- And silently wrong data is arguably worse than errors, but neither is good:

SET LANGUAGE français;

SELECT [April_20_1] = CONVERT(datetime, '04/20/2020'); -- Error!
SELECT [April_20_2] = CONVERT(datetime, '2020-04-20'); -- Error!
SELECT [April_20_3] = CONVERT(datetime, '20200420');   -- April 12th

-- Or NULLs, if you use [`TRY_CONVERT`]:

SET LANGUAGE français;

SELECT [April_20_1] = TRY_CONVERT(datetime, '04/20/2020'), -- NULL
       [April_20_2] = TRY_CONVERT(datetime, '2020-04-20'), -- NULL
       [April_20_3] = TRY_CONVERT(datetime, '20200420');   -- April 20th

-- And wrong data again, sometimes, if including time and the dates transpose correctly:

SET LANGUAGE français;

SELECT [April_12_a] = TRY_CONVERT(datetime, '2020-04-12 04:00:00'), -- December 4th!
       [April_12_b] = TRY_CONVERT(datetime, '2020-04-12T04:00:00'), -- April 12th
       [April_12_c] = TRY_CONVERT(datetime, '20200412 04:00:00');   -- April 12th


-- Or errors, if the days can't transpose correctly:

SET LANGUAGE français;

SELECT [April_20_a] = CONVERT(datetime, '2020-04-20 04:00:00'); -- Error!
GO

SELECT [April_20_b] = CONVERT(datetime, '2020-04-20T04:00:00'), -- April 12th
       [April_20_c] = CONVERT(datetime, '20200420 04:00:00');   -- April 12th