There are ways to use text in these calculations, but numbers are far simpler, particularly if you are planning to do any math with it. Which you are.
You can use the Month (date) function (results number) to pull the month number out of any existing date. Or in your case of moving up and down just enter a month number to start and work you button math from there.
You can use the inspector to attach a calendar to a date field and perhaps avoid all of that. Put the cursor in the field and a nice small calendar appears. Check it out.
You can use two fields--one to display the date and one that stores the month number that your script increments.
Define a number field named Month.
Define a calculation field as:
Date ( 1 ; Month ; Year ( get (CurrentDate ) )
specify "Date" as the return type.
Use data fromatting on your calculation field to display only the month name.
BTW, if formatting the date field as a pop up calendar as Jack suggested works for you, use that method. This alternative approach can save time when the dates you are entering are not usually the current month.
Phil, Thanks again for your solution, though I think you have the month and day switched in your calculation. This worked for me:
Date ( Month, 1, Year (get (CurrentDate)))