My wife has two monthly pensions. One she receives every calendar month, the other bifortnightly. However, I would go with Collins' putting the calendar month as the principal polyseme rather than the AHDEL's attempt to be all-inclusive resulting in the loose: 1 A unit of time corresponding approximately to one cycle of the moon's phases, or ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.