European traditions, the month starts when the
young crescent moon becomes first visible at evening after
conjunction with the Sun one or two days before that evening (e.g., in the
Islamic calendar). In
ancient Egypt the lunar month began on the day when the waning moon could no longer be seen just before sunrise. Others use calculation, of varying degrees of sophistication, e.g., the
Hebrew calendar or the
ecclesiastical lunar calendar. Yet others run from
full moon to full moon. Calendars count integer days, so months may be 29 or 30 days in length, in some regular or irregular sequence. In India the month from conjunction to conjunction is divided into thirty parts known as tithis. The date is named after the tithi ruling at sunrise. As the tithi is shorter than the day the date sometimes jumps.
common law, a "lunar month" traditionally meant exactly 28 days or four weeks, thus a contact for 12 months ran for exactly 48 weeks.
 In the United Kingdom, the lunar month was formally replaced by the
calendar month for deeds and other written contracts by the
Law of Property Act 1925 and for all other legal purposes by the
Interpretation Act 1978.