Bug with DTSTART as 31st of the month

[markhalonen]

I know this project is no longer maintained but wanted to document an issue in case it may be helpful for anyone else:

The RRULE

DTSTART:20250331T010100Z
RRULE:FREQ=MONTHLY;INTERVAL=6;WKST=MO

is not handled correctly, it does 1 date per year:

Switch it to 3/30 and it works as expected:

DTSTART:20250330T010100Z
RRULE:FREQ=MONTHLY;INTERVAL=6;WKST=MO

---

Testing some other last-day-of-month's:

• 01/01/2025 Succeeds
• 02/28/2025 Succeeds
• 03/31/2025 Fails
• 04/30/2025 succeeds
• 05/31/2025 Fails
• 06/30/2025 succeeds
• 07/31/2025 succeeds
• 08/31/2025 Fails
• 09/30/2025 Succeeds
• 10/31/2025 Fails
• 11/30/2025 Succeeds
• 12/31/2025 Fails

🤷‍♂️

[hysellr]

I don't believe this is an issue, but I may be able to help you understand what's happening here:

Your RRULE of:

DTSTART:20250331T010100Z
RRULE:FREQ=MONTHLY;INTERVAL=6;WKST=MO

This rule is every 6 months, starting on March 31. By default, monthly rules target the same Day of the Month (31 in your case) for each recurring event. The problem you're seeing is that 6 months later (September) does NOT have 31 days. Because the date is invalid, the event occurrence is ignored. 6 months after that brings you to 3/31/2026 which is a valid date. This results to ending up with 1 event per year, because the 2nd potential event for the year (9/31) is an invalid date.

You can actually replicate this same issue within Google Calendar event, which is a completely different implementation than this package. It is not unique to this rrule.js

Potential Solution: Use Relative Month Day ("Last Day of Each Month")

If you're looking to target the "Last Day of Each Month" in your interval, then you could use BYMONTHDAY=-1 in your RRULE. Using negative values allows you to count backwards from end of the number of days in the Month used in the interval.

Example:

DTSTART:20250331T010100Z
RRULE:FREQ=MONTHLY;INTERVAL=6;WKST=MO;BYMONTHDAY=-1

rule.toText(); // Result: "every 6 months on the last"

Which should produce the following event dates: 3/31/2025, 9/30/2025, 3/31/2026, 9/30/2026, and so on.

[TatsuUkraine]

Agreed with prev comment, it's not a bug, if you want to include all possible values you can either use solution described above, or following rule
BYSETPOS=-1;BYMONTHDAY=28,29,30,31

Where set of dates (28,29,30,31) is limited by your start date. Let's says if the start date is 29.01, then your bymonthday should be 28,29 (to use Feb last date as fallback). To include possible 02.28 date, if the start date is 30.01 then value will be 28,29,30. And if you have date like 28.01 then none of these additional statements are needed, since 28th exists in all months.