The following could would be expected to produce a 3-month output similar to
[[!calendar type="month" month="-1"]] [[!calendar type="month" ]] [[!calendar type="month" month="+1"]]
Behaviour: The 3rd entry doesn't show the next month, but the 1st month of the year (aka January).
Problem: Since there are no negative month numbers (unless someone starts with march because of Feb 29), –1 is interpreted correctly. Explicitely positive numbers aren't recognized as being relative. Possibly it is the numerical interpretation of the value, there is no difference between n and +n.
Solution: treat the value as string, check for a leading +, set a relativeMonth flag (which then also should happen on negative values, if it does not happen yet). If then it is set for the month in question, first calculate month_year and then go on as usual.
Idea: since i mentioned gcal earlier, how about some of the shorthanded sytax as "." for this, ".-" for previous, ".+" for next month together with its neighbours?