How Can We Store Repeating Dates in a Database While Accurately Handling Daylight Saving Time?

Storing Repeating Dates while Accounting for Daylight Savings Time

When creating a database to store events with repeating dates across multiple time zones, it's essential to consider the impact of Daylight Savings Time (DST) on your data. The current conversion methods from time zone to GMT may not account for DST, potentially leading to incorrect time conversions.

One suggested approach is to include a 'dst' field to indicate whether the dates were entered during DST. A method can be implemented to adjust the time by an hour as needed.

However, industry experts recommend a different approach.

Storing Dates in Local Time

Storing dates in local time is more appropriate for future local events, especially recurring ones. This approach avoids issues with DST affecting UTC conversions, resolving the "alarm clock problem" where events may trigger at the wrong time due to DST transitions.

To store this information effectively, consider including the following fields:

• Local time of the event (e.g., "08:00")
• Time zone the local time is expressed in (e.g., "America/New_York")
• Recurrence pattern (e.g., daily, bi-weekly, etc.)
• Next immediate UTC date and time equivalent
• Projected list of future UTC events (optional)

Calculating UTC Equivalents

The UTC equivalents should be recalculated regularly based on time zone database updates. It's important to have a plan to subscribe to these announcements and update your database accordingly.

Fall-Back Transitions

When an event falls during a fall-back transition (when DST ends), it's essential to decide if the event occurs on the first or second occurrence of the local time or both.

Alternative Approach

An alternative approach is to store dates in UTC using the following steps:

• Convert the local start date to UTC.
• Store the time zone ID.
• Reconvert the start date back to local time at runtime.
• Compute other recurrences based on the local time.

While this method works, it has drawbacks:

• Time zone updates can disrupt the schedule if they occur before the first instance of the event.
• "Floating times" following the user's time zone still require storing the original time zone information.
• It introduces unnecessary complexity.

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