1911 Encyclopædia Britannica/Month
MONTH (a common Teutonic word, cf. Ger. Mond, Du. maand, Dan. maaned, &c., and cognate with Lat. mensis, Gr. μήν, &c., in other branches of the Indo-Germanic family; all ultimately from the root seen in the word for the moon in nearly all those languages), originally the period between two returns of the new moon; generally called a lunar and sometimes a synodic or illuminative month. The anomalistic month is the mean time taken by the moon in passing from one perigee to the next; the sidereal month is the mean time in which the moon makes a circuit among the stars; the tropical month is the mean time in which the moon traverses 360° of longitude; the nodical or draconic month is the mean time taken by the moon in passing from one rising node to the next; the solar month is one-twelfth of a tropical year. The lengths of the various months are: synodic = 29.53059 days; anomalistic = 27.55460, sidereal = 27.32166, tropical = 27.32156, nodical = 27.21222, solar = 30.43685. (For the calendar months see Calendar.)
In law a month may mean either a lunar month, that is, a period of twenty-eight days, or a calendar month. At common law, “month” generally means a lunar month, although in mercantile matters it has been generally understood to mean a calendar month, but there is no general exception giving it that meaning in commercial documents. It can only have that meaning where according to the ordinary rules of construction a secondary meaning can be admitted (Bruner v. Moore, 1904, 1 Ch. 305). In bills of exchange or promissory notes month means a calendar month (Bills of Exchange Act, 1882, s. 14 ). Where a servant is engaged subject to a month's notice or payment of a month's wages month is interpreted as a calendar month (Gordon v. Potter, 1 F. & F. 644). In acts of parliament passed before the year 1850 month, unless otherwise specially interpreted, means lunar month, but in all acts passed since that date, month, unless words be added showing that lunar month was intended, means calendar month (Interpretation Act 1889, s. 3). In the rules of the supreme court and in the county court rules month means a calendar month. In mercantile contracts in computing the period of a month the day from which the time is to begin to run is excluded, but in sentences of imprisonment the day on which the sentence begins is included, so that the numerically corresponding day in the month in which the sentence expires would be excluded.