Recovery PR for #16173 after the fork branch was accidentally reset
during rewrite-cleanup.
Cherry-picked onto current `main`:
- fix(common/time_utils): correct fallback timestamp and ISO-8601
normalization
- fix(common/time_utils): preserve zero timestamps and mark regression
tests
- test(common/time_utils): make fallback assertions deterministic
Supersedes closed#16173 — same branch
`Harsh23Kashyap/fix/time-utils-edgecases`, rebuilt per @yuzhichang
recovery steps in
https://github.com/infiniflow/ragflow/pull/16173#issuecomment-4829663835
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Co-authored-by: Harsh Kashyap <harshkashyap@Harshs-MacBook-Pro.local>
Co-authored-by: Cursor <cursoragent@cursor.com>
### What problem does this PR solve?
Manage message and use in agent.
Issue #4213
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
- Add time utilities and unit tests
### Type of change
- [x] Refactoring
---------
Signed-off-by: Jin Hai <haijin.chn@gmail.com>
