[BUG] Flaky test: `CalcitePPLDateTimeBuiltinFunctionIT > testDateFormatAndDatetimeAndFromDays`

[Swiddis]

What is the bug?

CalcitePPLDateTimeBuiltinFunctionIT > testDateFormatAndDatetimeAndFromDays has failed multiple times for unrelated code changes:

• In Fix return types of MOD and DIVIDE UDFs #3513 -- https://github.com/opensearch-project/sql/actions/runs/14240445113/job/39908933119#step:4:1091
• In Fix: CSV handling of embedded crlf #3515 -- https://github.com/opensearch-project/sql/actions/runs/14250153623/job/39940768892?pr=3515#step:6:1094

Merged 2 days ago at the time of writing: 920f711

How can one reproduce the bug?
By accident.

What is the expected behavior?
The test should behave consistently.

What is your host/environment?
GitHub actions. Happens for at least Linux and Mac with both Java 21 and 23. Haven't seen it for Windows yet.

Do you have any screenshots?
N/A

Do you have any additional context?
N/A

[Swiddis]

Digging through the failing test and tracing several dependencies:

It looks like DATETIME('1984-04-12T09:07:42.000Z', '+08:00') and DATETIME('1984-04-12 09:07:42', '+00:00') inconsistently return null with Calcite (which causes two more nulls later). I can't pinpoint why at a glance, these should ideally be pure functions. Will leave it to the assignee.

Good case study for the "tests should fail for only one reason" principle :/

[xinyual]

Already raise the pr for it. #3517