InfoPath Dev

I am attempting to add months to a date that the User selects for a calibration record. I have the breakdown of Month/Date/Year into individual text boxes. To complete the calculation, I need to change the month from a text box to a number, and I have done that with a "Number" formula in the rules for the month text box. When I get the result, if it is under 10 months, it drops the preceding 0 and only shows the number of the month. When converting this back to a date format, it will not accept the single digit month. I am wondering how I can add a preceding 0 to the month (after converted to a number) so that I can convert it back to a proper date for "Next Calibration Date".

Sure, because numbers don't have preceding 0s. :) See the attached sample for one way to do this - I only included a few months in my dropdown, but there is a mix of single & double digits. Save the file locally, right click and select design, then preview and pick a month.