add `instance == Tm` and fix bug that `gmTime` could provide values

What does this MR do?

This fixes an issue with gmTime due to laziness of fields of Tm.

Related issues

Changes to public APIs

• instance == Tm was added

Author's checklist (required)

See CONTRIBUTING.md for the rationale behind these items:

• The commit history does not contain merges (use git rebase -i master if it does)
• Intermediate commits compile (use git rebase -i master if not)
• Newly added code follows the Platform code style
• Newly added code is documented
• If bugs have been solved, tests have been added
• Appropriate types have been used, especially in APIs
• If efficiency is part of the acceptance criteria of the issue, a benchmark is provided
• A changelog entry has been added if required. See CONTRIBUTING.md