Add time zone support

We should add time zone option that let’s the user specify which timezone they’d like to use.

Things this could affect:

• We could have one time zone for parsing data and one used for formatting labels and tooltips (e.g. timestamps are provided in UTC, but you’d like to display in Eastern timezone)
• Also used when we’re calculating major ticks / auto-ticks and try to align to the start of a minute, day, hour, etc. But when the start of the day is depends on the time zone.
• In some cases we may be able to determine the timezone the user would like to use from the labels they’ve specified (e.g. +0000)
• Unit tests are currently flaky because they depend on the timezone they’re being run in

Unfortunately JavaScript’s time zone support is pretty bad natively. E.g. there’s no way to create a Date object in a given timezone. Your only options are local system time or UTC time. We probably want to use only UTC dates or timestamps internally and convert to the proper time zone when needed for the cases listed above.

We would either need library support or would need to implement ourselves. In either case I think we should use Intl.DateTimeFormat APIs. We already get lots of complaints about the size of Moment.js (https://github.com/chartjs/Chart.js/issues/4303) and moment-timezone is even larger, so I think we should avoid using it.

[pvfpvf]

Just to add, the global fix moment.tz.setDefault(“UTC”); works for me. But if I use chartjs-plugin-zoom and zoom in, the X axis times immediately revert to local ones.

[benmccann]

Yes. I think we should switch from Moment to either Luxon or date-fns in order to fix this. We couldn’t switch to Luxon until Chart.js 3.0 since Chart.js currently exposes Moment in it’s interface. I have a pending PR to add timezone support to date-fns and have been waiting for it to be reviewed the past couple month, but the library authors are not very responsive.