This equation servers as an Equation-in-Equation (EiE) providing the author the ability to give a user the pull-down selection of month and then returning the integer value of that month.
In other words, you can drag-and-drop this into your equation code, add the following string to your equations Values field, and insert the variable you assign to that same list in the Values field and your input will be converted to integers corresponding to the month.
//January,February,March,April,May,June,July,August,September,October,November,December
def mo = args.mo
def moNum = execEquation("57a4d1b4-7794-11e5-a3bb-bc764e2038f2",mo) //Month Choice EiE
// Use moNum as the integer order of the months in your equation