Does the drift between two rules always keep growing?

Not necessarily at every single step, but once two differently defined rules diverge they essentially never reset back to zero difference, aside from rare coincidental alignments.

Can I compare more than two rules at once?

This tool compares exactly two rules side by side. For checking whether several independent schedules ever land on the same date, see the Recurrence Collision Finder instead.

Recurrence Drift Comparator

Compare a monthly calendar rule (same day of month) against a fixed every-N-days interval, both starting from the same date. See how far apart they've drifted at each occurrence.

Just need one schedule generated, not a comparison? See Recurring Date Generator.

Want to learn more?

Recurrence Drift: Why "Every Month on the 31st" Breaks

Worked example

A monthly rule anchored to the 1st and an every-30-days rule, both starting January 1, 2027, agree exactly on their first two occurrences, since January has 31 days and the gap between the two rules has not yet had a chance to show up. By the third occurrence they already differ by a full day, March 1 for the monthly rule against March 2 for the every-30-days rule, because months simply do not average out to exactly 30 days apiece. Run the comparison further, past a dozen occurrences, and the gap between the two schedules keeps widening rather than closing, since nothing in either rule corrects for the mismatch. This is precisely the kind of silent drift that is easy to miss for months at a time until two systems relying on the two different rules end up disagreeing about a real date.

Common mistakes

Assuming "monthly" and "every 30 days" are interchangeable phrasings for the same schedule. They only agree on occurrences where the elapsed calendar months happen to total exactly 30 days, which is not the general case.
Comparing only the first occurrence or two, where the two rules have not yet had enough time to diverge, and concluding the schedules match when they will not stay that way.

Limitations

Each rule here is generated independently and compared date by date. This does not model a real payment processor's or subscription system's own internal rounding or billing cycle rules, which may resolve the same underlying gap differently.

Frequently asked questions

Does the drift between two rules always keep growing?: Not necessarily at every single step, but once two differently defined rules diverge they essentially never reset back to zero difference, aside from rare coincidental alignments.
Can I compare more than two rules at once?: This tool compares exactly two rules side by side. For checking whether several independent schedules ever land on the same date, see the Recurrence Collision Finder instead.