In Interpretation of Statutes, Professor G.E. Devenish describes a calendar month as:
“A calendar month is a month according to the almanac or common calendar. A calendar month is therefore not a fixed number of days but varies according to the actual month concerned. A calendar month need not necessarily run from the beginning of a month. In ascertaining a certain number of calendar months the civil method of computation is used: the first day of the period is included and the last day excluded.
However the application of this definition is ambiguous. The term “calendar month” may be interpreted in two ways: either as a month as it appears on the calendar, (1 January until 31 January) or a month reckoned in terms of one day in a particular month until the corresponding day of the next month; thus for example, from the fourth day of March until the fourth day of April. Both possibilities arise in practice.”
From the above, it is clear that either period of notice is effective. If you want to make sure that notice must be from the 1st to the last day of a month, you must stipulate that, unambiguously, in your contracts of employment.