It somewhat depends on how the regularization date is actually calculated. Is it really 6 months, or is it actually 180 days?
Assuming your date is stored in A1:
An interesting thing about the months addition mode is that it seems to intelligently handle year wraparounds and end-of-month date mismatches, despite what the formula may appear to imply. Literally, the formula would seem to explicitly state that the YEAR and DAY value from A1 is used regardless of the month calculation. In reality, Excel will increment the year when needed (e.g.: your date is in July or later), and it will also account for months lacking a 31st (or 29th or 30th) by pushing the date into the next month.
A perfect example of this would be setting the start date to August 31st. Depending on whether the next year is a leap year, your regularization date would be either March 2nd or 3rd of the next year instead of the very erroneous February 31st of the same year.
More information on adding dates can be found on Microsoft's support site. Though that link is particularly in reference to Excel 2003, the same formulas work up to 2013 just fine.