Since each lunation is approximately ​291⁄2days (29days, 12hours, 44minutes, 3seconds, or 29.530588 days), it is common for the months of a lunar calendar to alternate between 29 and 30days. Since the period of twelve such lunations, a lunar year, is only 354days, 8hours, 48minutes, 34seconds (354.367056days), purely lunar calendars lose around 11days per year relative to the Gregorian calendar. In purely lunar calendars like the Islamic calendar, the lack of intercalation causes the lunar months to cycle through all the seasons of the Gregorian year over the course of a 33lunar-year cycle.