Daylight Saving Time Bug – Recurring Events Shift by One Hour

[compsoul]

The Calendar plugin incorrectly handles the transition from Standard Time to Daylight Saving Time. During the week of the DST change (last weekend of March), all recurring events are automatically shifted by one hour.

Steps to Reproduce:

1. Create a recurring weekly event set for Tuesdays, from 11:10 to 12:00.
2. Observe the event time:

• In the week before the DST change (e.g. March 25, 2025) — the event correctly shows 11:10 – 12:00 (see attached screenshot).
• In the week after the DST change (e.g. April 1, 2025) — the same recurring event now appears as 12:10 – 13:00, even though no change was made to the recurrence settings (see second screenshot).

Expected Behavior:
Recurring event times should remain fixed according to the local time zone and should not shift due to the DST transition. An event set for 11:10 should continue to appear at 11:10 after the clock change, unless the user explicitly updates it.

Suggested Fix:
Please review how the plugin handles time zones and daylight saving adjustments in recurring events. It's likely the system is using UTC-based calculations without adjusting for local DST rules. Consider using timezone-aware libraries (e.g. moment-timezone) and ensure recurring rules respect TZID parameters correctly.

[brlumen]

@compsoul , Thanks for the bug report. I will definitely check the operation of the algorithm for recurring events, taking into account daylight saving time.