The February and Leap Year Edge Case
If a cron job is scheduled with `0 0 29 2 *`, it will execute exactly at midnight on February 29th. This means the job will run only once every four years.
Daylight Saving Time (DST) Shifts
DST transitions are the source of major scheduler bugs:
1. **Spring Forward (Lost Hour)**: When the clock skips from 2:00 AM to 3:00 AM, any job scheduled during that lost hour (e.g., `30 2 * * *`) **may not run at all** depending on your scheduler engine.
2. **Fall Back (Duplicate Hour)**: When the clock repeats the hour from 1:00 AM to 2:00 AM, any job scheduled during that repeated period **may run twice**.
Mitigation Strategy
For mission-critical production jobs, always configure your server system clocks and application cron schedulers to run on **Coordinated Universal Time (UTC)**, which has no daylight saving shifts.