WIP: datetime filtering fixes

[sc1f]

This draft PR will contain fixes for Perspective's datetime filters, specifically the issues mentioned in #1242 (and any others that may come up) - I'm using this draft PR as a place for notetaking/progress-checking and commentary.

Tasks:

• Fix how the datestring is being converted to timestamp
  • == filter compares the Unix timestamps - conversion not working properly?
• Improve UX for datetime filters
  • Make datestring format more lenient?
  • Add a hint to what formats are supported/make the default format more intuitive?
• Codify the errors that are happening, so we know exactly what to fix
• Enable programmatic filters using Date() values, not just strings
  • Implemented already, although needs more tests

[sc1f]

Issue with == filters on datetimes:

Because Perspective stores datetimes as Unix timestamps (ms since epoch), it performs the == filter by matching timestamp values. Perhaps filter values are being converted to timestamps in local time according to the browser, but Perspective (especially with arrow format data) is storing the timestamp in UTC.

[sc1f]

@texodus this change fixed date filtering from:

to:

I think this issue was around for a while (earliest version I tested was 0.4.8, far before the date parser rewrite). What I think is happening - Date.getTime() always uses UTC, so the timestamps stored by Perspective are in UTC, and then re-localized to local time when they are passed into new Date() when output. However, this function (and its previous implementation, jsdate_to_t_date) pulled the ymd values as local time, which then causes a re-localization on top of an already localized datetime - thus the issue happening in the screenshot above. I would like to verify this with a proper test suite akin to what we have in the Python tests - that will be coming as part of this patch.

What I am unsure about - if datetime values are stored as UTC timestamps and correctly localized, then why does == on a datetime still show an offset:

This behavior has also been around since 0.4.8.