The number of days in a month is measured in ones

The synodic month, or complete cycle of phases of the Moon as seen from Earth, averages 29.530588 mean solar days in length (i. e., 29 days 12 hours 44 minutes 3 seconds); because of perturbations in the Moon's orbit, the lengths of all astronomical months vary slightly. The sidereal month is the time needed for the Moon to return to the same place against the background of the stars, 27.321661 days (i. e., 27 days 7 hours 43 minutes 12 seconds)