[mjevans]

No, you should be able to express it the way I mentioned in other examples: Base .plus( N months ) where N is whichever month after the reference you want.

.plus .plus .plus isn't correct because "x months" doesn't have a fixed size. You are NOT saying Base .plus(30 days), NOR are you saying Base .plus(4 weeks) ((which BTW, I'd expect to stay on the same weekday)). You're incrementing by an unstable value.

[saurik]

This doesn't work either as it wouldn't work correctly if started in a month with less than 31 days. To represent "the last day of the month" you really need to be subtracting one day from the first day of the next month, full stop: any solution working off of these weird rounding rules is going to be unreliable.