Since each lunation is approximately 29+1⁄2 days,[1] 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 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days),[1] purely lunar calendars are 11 to 12 days shorter than the solar year. 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 33–34 lunar-year cycle.