Ive noticed today (the 31st) that the + month doesnt work if the current day is on the 31st. The calculation seems to skip over other months.
Im thinking either:
A) the calculation only will move to the next month that also has 31 days
B) the calculation adds 30 days and takes the month at that time. The problem is you can skip over february or hit the same month twice.
By subtracting 1 month today Ive been getting the following results
Current date (format as mmm): May
Current date +month -1: May
Current date +month -2: Mar
Current date +month -3: Jan
Also below is a screenshot of some buttons that show the previous month, this month and next month using the (+month) setting. It shows May, May, July on May 31st.