You're not going to find the answer varying. A calendar month is the month you see on the calendar, such as the beginning of July to the end of July.
If another period of time is specified, then it will be clear, such as the preceding 30 days. In that case, look back 30 days. If you're restricted to X amount of flying in a calendar month, or a calendar year, then its the month or year as you see it on the calendar. Don't read any more into the limitation than is already spelled out by the term itself.