FA-76885 / Calendar recurrence rules / Member archive

Monthly BYMONTHDAY expansion with negative and invalid days: dedupe coinciding · case 05

BYMONTHDAY=31,-1 emits the 31st twice in long months.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

FREQ=MONTHLY with a BYMONTHDAY list. Positive values count from the first day, negative values from the last day (-1 is the last day). A value that does not exist in a month is skipped for that month, never clamped. Months step by interval from the dtstart month; instances before dtstart are dropped; coinciding values yield one instance. Return the first count ISO dates in chronological order.

Why this case matters

Recurring calendar series are expanded into concrete instances for display, reminders and conflict checks.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
31 and -1 coincide	`["2024-01-31", "2024-01-31", "2024-02-29", "2024-03-31"]`	`["2024-01-31", "2024-02-29", "2024-03-31", "2024-04-30"]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.