How Can We Efficiently Store Repeating Events in a Database While Accounting for Daylight Saving Time?

Storing Repeating Dates with Daylight Savings Time in Mind

When storing events in a database, the challenge arises when dealing with repeating events across multiple time zones, particularly due to Daylight Savings Time (DST). DST can lead to inconsistencies in time conversions when events span different seasons, affecting the recurrence schedule.

Current Methods

Current methods involve converting date/times to GMT before saving and reverting them to their respective time zones for display. The time zone is typically stored in a VARCHAR field, such as "America/New_York."

Recurring Event Complications

With the introduction of repeating events, the user defines a start date and recurrence pattern. However, DST can disrupt the schedule as it alters the time conversion difference between GMT and the local time zone. For example, an event starting in July with a monthly recurrence may encounter a DST transition, resulting in different time adjustments depending on the month.

Proposed Solution

One proposed solution involves storing a tinyint(1) flag for DST in conjunction with start/end dates. This flag would indicate whether the dates were entered during DST. A method could then be applied to adjust the time by an hour if necessary.

Alternative Approach Using Local Time

An alternative approach is to store the following information:

• Local time of the recurring event
• Time zone
• Recurrence pattern
• Next immediate UTC date and time equivalent
• Projected future events (optional)

This approach mitigates DST-related issues by basing the recurrence schedule on the local time. However, it introduces the challenge of managing time zone updates, as time zone database updates can affect future event calculations.

Additional Considerations

DST transition handling: When an event is scheduled for a local time that occurs during a DST fall-back transition, it's important to determine whether it occurs on the first or second instance of the time, or both.
Floating time: For floating times that should adapt to the user's current time zone, the original time zone of the event still needs to be stored when using UTC-based scheduling.
Additional complexity: Using UTC-based scheduling with time zone adjustments introduces complexity and is generally reserved for situations where adapting existing UTC-only schedulers is necessary.

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