[AIRFLOW-8875] fix Dag Run UI execution date with timezone cannot be saved issue

[realradical]

This PR is to address the following bug: #8842
Airflow 1.10.10 displays datetime with timezone on the web UI. Currently, when user tries to create a dag run on the UI, it gives invalid DateTime error which prevents the dag run being created.

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[realradical]

@ashb the quarantined CI build job fails.. any idea?

[ashb]

Looking better @realradical!

Can you test that changing the timezone in the UI updates this field while the page is open? (If it doesn't it means it's missing an html class)

I also think that we should test more than one timezone, and for completeness, test what happens when the form is submitted without a TZ (it should use the "default" configured TZ. So we should also use @conf_vars for that test to change the default for that one test to not be UTC.)

[realradical]

@conf_vars

@ashb I tried changing the timezone in the UI and it works!

I used @conf_vars({("core", "default_timezone"): "America/Toronto"}) in the test without timezone. However, the timezone setting used in timezone.make_aware utility function is initialized before the config overriding in my test. https://github.com/apache/airflow/blob/master/airflow/settings.py

Personally I don't think we need to cover this case, the frontend should make sure that the datetime submitted always with a timezone.

[ashb]

@realradical I think the backend change you have here is good! With it it should accept any datetime that the UI displays, but without it it would only accept TZ in the current TZ.

i.e. the change as you had it meant you could copy-and-paste a date-time with TZ and have it work, so I'd still think this is a good change to have.

[realradical]

@ashb i accidentally closed the pr. will create another one now. sorry about it