variable: fix outdated time shift for variable with TypeTime (#39720)

[ti-chi-bot]

This is an automated cherry-pick of #39720

Signed-off-by: YangKeao yangkeao@chunibyo.icu

What problem does this PR solve?

Issue Number: close #39719

Problem Summary:

The time shift for a time zone can differ in the history. For example, Asia/Shanghai is +0805 in the history. If the user set variable without timezone, and TiDB equipped it with local time zone by default. It'll result in an unexpected behavior.

What is changed and how it works?

Use the now year, month and day. Actually any near enough date is fine (as the timezone shift doesn't change frequently, right?). I still use the current date to make it more robust.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

Fix the issue that setting time variables (for example, `tidb_auto_analyze_start_time`) without explicit timezone will give an outdated time shift.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bb7133
• qw4990

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[bb7133]

LGTM

[hawkingrei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 802f489

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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[YangKeao]

/run-unit-test