[speg]

You just increment the month. March 20 m+1 is April 20.

If the date is invalid, like January 30 m+1 then you fallback to next valid date, February 28/29.

[blipmusic]

Well, that’s not “the month” at all, though. At least not something I’d use outside of getting a rough idea for setting the next meeting, and absolutely wouldn’t use in code (but it’s probably fine for a todo - I’m complicating things, sorry ;-))

Edit: but perhaps that’s the custom used at many places and I’m just not aware.

[johntash]

A few people/companies I've worked with avoid using the last few days of the month for "monthly" things.

For example, "every month on the 15th", "every month on the 1st", etc. It makes it easier to figure out compared to "every 30 days" or "every month on the 30th, unless there are less than 30 days in the month"