English common law has centuries of experience on this issue, and has developed some settled principles that are applied by the courts.
Prior to 1925, references in contracts to months were interpreted as
lunar months. There are 13 lunar months in a year. By section 61
of the Law of Property Act 1925, references in contracts to months
are to be interpreted as calendar months, unless the context
Calendar months are counted from a specified date. It is not necessary to consider the period from the 1st of a
month to, say, 31st (unless the notice period starts on 1st of a
month). This contrasts with the usual understanding of calendar
year, which tends to be thought of as the period from 1st January to
The “corresponding date rule” requires that one looks
at the same date in the diary the relevant number of months ahead.
Thus a period of 3 months from 20th April expires on 20th July. In
the case of Dodds v Walker  1 WLR 1061, in the Court of
Appeal, Templeman LJ said:
"…if an act is authorised to be performed on any arbitrary day in any month of the year, then one month elapses on the corresponding day of the next month, provided that the day of the act itself is excluded from computation."
In the same case, in the House of Lords, Lord Diplock said:
"The corresponding date rule is simple. It is easy of application. …all that the calculator has to do is make in his diary the corresponding date in the appropriate subsequent month."
In the same case, Templeman LJ clarified that if the period expired on a date that did not exist in a particular month (eg 31st February), then the period would expire on the last day of that month.
Law Stack Exchange is for educational purposes only and is not a substitute for individualized advice from a qualified legal practitioner. Communications on Law Stack Exchange are not privileged communications and do not create an attorney-client relationship.