[FLINK-28929][table] Add built-in datediff function.

[liuyongvs]

What is the purpose of the change

Add built-in datediff function.

[flinkbot]

CI report:

• f810738d4b609c38319602abce7b5f3b75dcc60d Azure: SUCCESS

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[snuyanzin]

It would be great if PR message matches PR template. Once is is stated that it could calculate number of days between 2 datetimes it would be nice to see some clarification for corner cases like one datetime in summer time and another datetime in a process of movement to winter datetime (time is same for both) or what if 2 datetimes are with different timezones

[liuyongvs]

@snuyanzin i don't find timestamp with timezone type to construct it

[snuyanzin]

from one side yes there was a jira issue to support it from another side there is a list of functions which could return timestamp with timezone e.g. convert_tz from https://nightlies.apache.org/flink/flink-docs-release-1.15/docs/dev/table/functions/systemfunctions/#temporal-functions

the point is that if we are talking about seconds/minutes/hours then we should expect something like different timezones or summer/winter time

[liuyongvs]

i will create a issue to support the timestamp with time zone with all kind of this temporal function in TimeFunctionsITCase @snuyanzin , whay do you think? then we just focus on this function.

[liuyongvs]

and do you have time to review this https://github.com/apache/flink/pull/20544 and https://github.com/apache/flink/pull/20546 ? and the ci is not green because of kafka end to end test, which is not relate to my pr

[snuyanzin]

does it make sense in that case to support only dates without seconds/minutes/hours?

[liuyongvs]

does it make sense in that case to support only dates without seconds/minutes/hours?

yeap.

[MartijnVisser]

@liuyongvs Please make sure that you use and fill in the PR template. Now it requires scanning through the code, to see if tests are added etc.