Handling Daylight Saving Time in Azure Function App Timer Trigger with Python

Описание: Learn how to manage Azure Function App timer triggers in Python to handle daylight saving time dynamically by combining fixed schedules with runtime checks.
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This video is based on the question https://stackoverflow.com/q/79186648/ asked by the user 'LT_AKR' ( https://stackoverflow.com/u/21398518/ ) and on the answer https://stackoverflow.com/a/79503704/ provided by the user 'JohTok' ( https://stackoverflow.com/u/21122975/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Challenge: Azure Timer Trigger and Daylight Saving Time

Azure Functions Timer triggers use CRON expressions defined as literal strings for their schedules. This design means you can't dynamically change the schedule at runtime to account for daylight saving time (DST) shifts directly in the CRON.

When your function must run at a local time that shifts due to DST (e.g., 23:00 in Stockholm), a fixed UTC-based CRON triggers at the wrong local time after the DST change.

Why Can't We Use Dynamic CRON in Python Azure Functions?

The @ app.schedule() decorator in Python Azure Functions expects a literal CRON expression string. Even interpolating or assigning variables with CRON strings won't cause the trigger to adapt dynamically at runtime.

Practical Solution: Fixed Multi-Time Schedule + Runtime Filtering

Since Azure doesn't allow dynamic CRON schedules, a common workaround is:

Schedule your function to run at multiple candidate times that cover both DST and standard times.

Inside the function, check the current local time.

Exit early if the function is running outside the intended local time window.

Step 1: Define a Helper Function for Local Time Check

[[See Video to Reveal this Text or Code Snippet]]

This function defines a 20-minute window to trigger around 23:00 local time, allowing small timing differences.

Step 2: Schedule Multiple Times Covering DST and Standard Time

Schedule your function at two close UTC times that align with 23:00 local time in both DST and standard time:

[[See Video to Reveal this Text or Code Snippet]]

The CRON schedule runs the function at both 21:00 and 22:00 UTC.

At runtime, should_run_script() ensures only one execution proceeds at the correct local time.

Benefits of This Approach

Reliability: Uses Azure Functions' supported CRON format.

Simplicity: Avoids complex infrastructure to alter schedules dynamically.

Efficiency: The minor overhead of the "dry" runs (which exit early) is small.

Flexibility: Adaptable for any timezone and any DST rule.

Summary

To handle daylight saving time in Python Azure Function timer triggers:

Use fixed CRON strings covering all possible trigger times.

Check the current local time inside your function.

Run your core logic only if the check passes.

This method is a pragmatic way to work around Azure Functions' static CRON limitations while ensuring your tasks align with local time regardless of DST shifts.