Date&Time improvements

[qoega]

• Add getTimeZone function. Trivial, but can help user to answer question about timezone in all cases - toTypeName for DateTime without timezone does not answer this question and you need to read the docs

getTimeZone(now()) = timezone()

• Add modifier in formatDateTime to print 'Time offsets from UTC' in following formats "+02:00","+0200", or "+02". Note: +00:00 should be printed with no offset.

• Add parseDateTimeBestEffort description. At least describe arguments. Probably move out of type conversions.

• dateDiff documentation is incomplete. Probably we should define function behavior more strictly.

[alexey-milovidov]

Note: +00:00 should be printed with no offset.

ISO 8601 also allows Z.

[hhell]

getTimeZone

A bit of a hack, but it can be implemented by
if(toTypeName(val) like 'DateTime%(%', splitByChar('\'', toTypeName(val))[2], NULL))

Example:

select
    toDateTime(now(), 'Europe/Moscow') as val,
    if(
        toTypeName(val) = 'DateTime',
        timezone(),
        if(
            toTypeName(val) like 'DateTime%(%',
            splitByChar('\'', toTypeName(val))[2],
            NULL))
    as val_tz
format JSONCompact

[grundrauschen]

It would also be great to add a modifier to formatDateTime to print subsecond values when used in conjunction with DateTime64. Together with the timezone offset from above I would then be able to get a full ISO-8601 timestamp from DateTime64 like 2020-11-27T14:54:10.123456Z.

[alexey-milovidov]

The function has been added under timezoneOf name.

[alexey-milovidov]

Closing in favor of #40103.