Today is 31 January. I added a task and set a due date with textual form "in 1 month". The expected result was either the last day of February, or some of the first days of March.

To my surprise RTM proposes... 31 March!

Doing some experiments it gives the same "off by one month" result when setting due date "in X months" when X is odd.

My guess is the algorithm works as follows:
1. Using some calendar library, add X months to the current date.
2. Set the day in the resulting date to the day of the current date.

which apparently fails, when there resulting (+X) month has fewer days than the current day of the month is.

Interestingly, the bug occurs through web access, but not over API.