Fix 'AT TIME ZONE' issue near DST change time with DATETIME2 datatype conversion

[Anikait143]

Description

1. Issues

• Convert function when converting to datetime2, datetime, smalldatettime with given timezone using AT TIME ZONE gives wrong output near DST change time.

2. Changes made to fix the issues

• Fix timestamp value to consider it in the given timezone instead of UTC for datetime2, datetime, smalldatettime datatypes. This fixes the issue and also makes it independent of local timezone setting.

# Input
1> SELECT CONVERT(DATETIME2(0), '2022-03-27T02:00:00', 126) AT TIME ZONE 'Central European Standard Time';
2> GO

# previous output (wrong output)               

---------------------------------------------
          2022-03-27 02:00:00.0000000 +02:00

# current output (correct output)

---------------------------------------------
          2022-03-27 03:00:00.0000000 +02:00                                                                                                                                                                                                                                           

# previous output (wrong output)
1> SELECT CONVERT(DATETIME2(0), '2022-03-27T01:01:00') AT TIME ZONE 'Central European Standard Time';
2> GO
timezone 
---------------------------------------------
           2022-03-27 01:01:00.0000000 +02:00

(1 rows affected)

# current output (correct output)
1> SELECT CONVERT(DATETIME2(0), '2022-03-27T01:01:00') AT TIME ZONE 'Central European Standard Time';
2> GO
timezone 
---------------------------------------------
           2022-03-27 01:01:00.0000000 +01:00

(1 rows affected)

• Added Test Cases for this issue

cherry-picked: #3343
Task: BABEL-5513
Authored-by: Anikait Agrawal [email protected]
Signed-off-by: Anikait Agrawal [email protected]

Test Scenarios Covered

• Use case based - YES

• Boundary conditions - YES

• Arbitrary inputs -

• Negative test cases -

• Minor version upgrade tests -

• Major version upgrade tests -

• Performance tests -

• Tooling impact -

• Memory tests -

• Client tests -

Check List

• Commits are signed per the DCO using --signoff

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[coveralls]

Pull Request Test Coverage Report for Build 12593663257

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 74.879%

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Files with Coverage Reduction	New Missed Lines	%
contrib/babelfishpg_tds/src/backend/tds/tdsutils.c	1	73.66%

Totals
Change from base Build 12584870045:	0.001%
Covered Lines:	46736
Relevant Lines:	62415

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💛 - Coveralls