Recurring Events and Daylight Saving Time
Last Updated: 14d
Daylight Saving Time is handled differently across different regions. The date for transitioning in and out of Daylight Saving Time can vary from place to place. Additionally, some areas transition at midnight, while other transition at 2am. Still other areas don't recognize Daylight Saving at all.
This can create some confusion when it comes to the topic of recurring events. If you have an event that occurs at a specified time every week, whose DST rules should it follow? And more importantly, what happens to that repeating event when you are traveling in a region that handles DST differently (or not at all)?
When you create a recurring event, part of the standard spec for how recurrence rules are stored is that they include the " creation time zone". This means that when calendar systems share data (for instance, if you are syncing calendar events between Google and Pocket Informant), we inform each other about a recurrence rule's creation time zone. When a calendar (such as Pocket Informant, Apple Calendar, Google Calendar, etc) expands a repeating event, it follows the DST rules of the "creation" time zone - not the current time zone.
There is good reason for this - especially in the world of shared calendars. Consider the following example:
Suppose you do business with a company in New York (Eastern Time which does acknowledge DST), but you live in Arizona (which does not change for DST at all). This company has a weekly conference call every Wednesday at 10am EST...and this conference call is on a shared calendar that you and they both have access to. For you, this call will normally occur at 8am. However, when New York changes their clocks for DST, you do not. So during this time (from March to November), you are experiencing this conference call at 7am while they still experience the same call at 10am like they always have. You will see this reflected on your calendar as well - if you are in Arizona, all occurrences of this event outside of DST will show as being at 8am. However, during the date range from March to November while New York changes for DST, your calendar will show these event occurrences happening at 7am just like you'd expect. For everyone in New York, their calendars will all show the same event always occurring at 10am.
This all works because calendars use the creation time zone's rules for DST.