§1. The initial day (a quo) is not computed in the total unless its beginning coincides with the beginning of the day or the law expressly provides otherwise.
§2. Unless the contrary is established, the final day (ad quem) is computed in the total which, if the time consists of one or more months or years, or one or more weeks, is reached at the end of the last day of the same number or, if a month lacks a day of the same number, at the end of the last day of the month.
Source: Code of Canon Law (Vatican.va)