Cron vs Interval Scheduling
Wall-clock cron expressions versus fixed intervals—compare drift, timezone handling, and predictability for scheduled jobs.
Overview
Cron and intervals fail in opposite ways. Cron gives you exact clock times and therefore inherits every timezone and daylight-saving edge case, including the hour that happens twice. Intervals ignore the calendar entirely, which is robust but drifts after each restart and cannot express “run on the first business day of the month”.
Cron schedule
Open Cron ExplainerPros
- Runs at predictable wall-clock times
- Expresses business calendars naturally
- Universally understood by operators
Cons
- Daylight saving shifts cause skips or repeats
- Field syntax is easy to misread
Fixed interval
Pros
- Simple “every N minutes” mental model
- Immune to timezone and DST surprises
- Easy to jitter to avoid thundering herds
Cons
- Start time drifts after restarts
- Cannot express “first Monday of the month”
Comparison table
| Aspect | Cron schedule | Fixed interval |
|---|---|---|
| Anchored to | Wall-clock time | Elapsed time |
| DST behaviour | Can skip or double-run | Unaffected |
| Calendar rules | Supported | Not supported |
| Best fit | The job must align with clock time or a calendar | The job just needs to run regularly |
Recommendation
Use cron with an explicit timezone for calendar-bound work such as reports and billing, and intervals for maintenance loops where only frequency matters. Read any unfamiliar expression in plain language before deploying it.
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Frequently asked questions
- How do I avoid daylight-saving problems?
- Schedule in UTC when the exact local hour does not matter, or pin an explicit timezone and avoid the one-to-three a.m. window where transitions occur.
- Why did my cron job run twice?
- During a backward DST transition the same local hour repeats. Make the job idempotent so a duplicate run is harmless.
- What pushes someone toward Cron schedule instead of Fixed interval?
- Cron schedule wins when the job must align with clock time or a calendar. The practical upside is that runs at predictable wall-clock times, and expresses business calendars naturally. The trade-off to watch is that daylight saving shifts cause skips or repeats.
- When does Fixed interval beat Cron schedule for the same job?
- Reach for Fixed interval when the job just needs to run regularly. It gives you simple “every N minutes” mental model plus immune to timezone and DST surprises, at the cost that start time drifts after restarts.
- Can ToolHub help me try Cron schedule and Fixed interval before I commit?
- Yes. The tools linked from each side of Cron vs Interval Scheduling run in your browser, so you can exercise Cron schedule and Fixed interval with sample data without uploading anything.