How a Daylight Saving Time Bug Caused Biweekly Production Failures

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A software developer encountered a recurring production bug where a report generation job failed every other Thursday at 2 AM, but worked flawlessly in local testing. Investigation revealed the root cause was Python's naive datetime object being shifted across a daylight saving time boundary on a US/Eastern-timezone server. Because the datetime lacked timezone awareness, adding 14 days during a DST transition caused the resulting due date to be off by one hour, breaking compatibility with downstream services. Using a structured debugging framework of Observe, Hypothesize, Test, and Learn, the developer isolated the issue by forcing the job to run with a known DST transition date and logging raw datetime values. The fix involved treating all datetime objects as timezone-aware from the point they entered the system, which eliminated the intermittent failures entirely.

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ProgrammingDEV Community ·

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