[Bug]: 3.0-dev lacks time and date compatibility fixes

[LeftHandCold]

Is there an existing issue for the same bug?

• I have checked the existing issues.

Branch Name

3.0-dev

Commit ID

d1e78d8

Other Environment Information

- Hardware parameters: N/A (branch-compare audit)
- OS type: Darwin
- Others:
  - Compared `base/main` vs `base/3.0-dev`
  - Audit window: 2025-10-01 .. 2026-04-24

Actual Behavior

3.0-dev is still missing a cluster of main-only MySQL time/date compatibility fixes. The highest-priority missing work covers fractional-second precision for TIME/DATETIME/TIMESTAMP, MySQL 8.0-aligned datetime behavior, time arithmetic (TIMEDIFF / TIMEADD / TIMESUB / TIMESTAMPADD / TIMESTAMPDIFF), CURTIME, and related time-formatting / timezone-sensitive behavior.

Expected Behavior

3.0-dev should match main for user-visible time/date/datetime/timestamp behavior, especially where main already aligned semantics with MySQL 8.0.

Steps to Reproduce

1. Compare `base/main` and `base/3.0-dev` for PRs `#22708`, `#23112`, `#23150`, `#23161`, `#23051`, `#22929`, `#23761`.
2. Verify missing surfaces on `3.0-dev`:
   - `git grep -n 'TIMESTAMPADD' base/3.0-dev -- pkg/sql/parsers/dialect/mysql/mysql_sql.y` -> no match
   - `git grep -n 'ADDTIME' base/3.0-dev -- pkg/sql/plan/function/list_builtIn.go` -> no match
   - `git cat-file -e base/3.0-dev:test/distributed/cases/function/func_datetime_curtime.test` -> missing
   - `git cat-file -e base/3.0-dev:test/distributed/cases/function/func_datetime_edge_cases.test` -> missing
3. Exercise representative SQL on `3.0-dev`, for example:
   - `select curtime();`
   - `select timediff('10:00:00', '09:59:59');`
   - `select timestampadd(second, 1, '2025-01-01 00:00:00');`
   - `select timestampdiff(day, '2025-01-01', '2025-01-03');`
   - `select get_format(date, 'USA');`
   - TIME/DATETIME/TIMESTAMP columns with scale 0/3/6 and boundary values
   - `LOAD DATA` on TIMESTAMP values under a non-default session timezone.

Additional information

Main-only reference commits:

• 4ae1bb45d8 Enforce fractional-second precision end-to-end for TIME/DATETIME/TIMESTAMP, align with MySQL 8.0, add comprehensive tests (#22708)
• b494d87c3f fix: several datetime-related issues and improves MySQL 8.0 compatibility: (#23112)
• bf7ae49546 fix: time-related mysql compatibility (#23150)
• 2377651895 fix: TIMESTAMPADD and TIMESTAMPDIFF compatibility with MySQL (#23161)
• 9f71c57837 Adding curtime. #22931 (#23051)
• 72791e25a1 Fix: LOAD DATA should use session timezone for TIMESTAMP parsing main (#22929)
• 50a63af528 fix get format date (#23761)

Non-time function/window parity is tracked separately in #24185.