Timezone support

[gajus]

Is setting a timezone not supported?

[erkstruwe]

After having implemented a lot of functionality in my application based on this repo I found out today that something like

DTSTART;TZID=/Europe/Berlin:20221029T080000
RRULE:FREQ=DAILY;UNTIL=20221031T080000

is actually not supported by this repo.

This is usually needed to cater for daylight saving in a particular time zone. In this case, you would expect occurrences to always be at 8AM German Time. This, however, corresponds to 6AM UTC on Oct 29th 2022, but 7AM UTC on Oct 30th 2022. Thus, interpreting times as UTC does not work here.

However, after playing and reading around quite a bit I came up with a solution that worked for me:
Consider datetimes in the database as "floating" instead of UTC and do not store any time zone information in it. "Floating" time zones are actually a thing (https://icalendar.org/CalDAV-Access-RFC-4791/7-3-date-and-floating-time.html).

"_rrule"."occurrences" for the ruleset

DTSTART:20221029T080000
RRULE:FREQ=DAILY;UNTIL=20221031T080000

yields this result in my JavaScript application code:

2022-10-29T08:00:00.000Z
2022-10-30T08:00:00.000Z
2022-10-31T08:00:00.000Z

Leveraging the parseFromTimeZone function from date-fns we can adjust them for time zone and daylight savings at the same time, though, and get the expected results:

parseFromTimeZone(occurrence.toISOString().slice(0, -1), { timeZone: "Europe/Berlin" })
// 2022-10-29T06:00:00.000Z
// 2022-10-30T07:00:00.000Z
// 2022-10-31T07:00:00.000Z