FileMaker's later versions (many years) can help with a "date". Which is that if your calculation uses a date, to make it later (or earlier), it will help setting the correct month, and year, for that calculation. I think you've seen that (likely all newbies take it for granted :-). I'm just saying it as it happens in this calculation.
The result in trying for with this calculation is do give me the Year & Month (only, not a date). It is a number Type of result. Because of this it adds a "0" to the month number, if needed; i.e., next month is: 201412; next month after that will be 201501 (first month of next year). [ It automatically use 1 for the month, and one more for the year.]
If you have a calculation like the above for one side of a relationship, and similar calculation in the target table, you can easily match on a month (with just equals "=", not needing <> on (multiple) dates. [ It's also useful for a scripted Find on a month.]
Let ( [
d = Get(CurrentDate);
// d = GetAsDate ("12/1/2014"); // test for next year
date_next_month = Date ( Month (d) + 1; 1; Year (d) );
month_next = Right ( "0" & Month (date_next_month); 2);
year_next = Year ( date_next_month )
year_next & month_next
[ The 3rd line above, "// d =" etc., does not run. It is their so I (or you) can tern it one once, in order to see that the above (without it) will return the correct result next month (12).]
P.S. This is the way I do it. Others may use date results for such relationships, etc.; both work, but I find the above simpler (to my brain).