Since each lunation is approximately  days (29 days, 12 hours, 44 minutes, 3 seconds, or days), it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year, is only 354 days, 8 hours, 48 minutes, 34 seconds ( days), purely lunar calendars lose around 11 to 12 days per year relative to the Gregorian calendar. In purely lunar calendars, which do not make use of intercalation, like the Islamic calendar, the lunar months cycle through all the seasons of a solar year over the course of a 33 lunar-year cycle.