Chinese Calendar
The traditional Chinese Calendar is based on the calculated positions of the sun and moon. A new month will always begin on the day of a new moon, and will have either 29 or 30 days, giving each month an average of 29.5 days in a year. A normal year contain 12 months, giving 354 days, which is around 11 days short of the 365 days, 6 hours, 9 minutes and 9 seconds required for Earth to make one revolution around the Sun. Thus, every 2 or 3 years, an additional month will be added to the normal year. In any given period of 19 years, there will be 7 years with 13 months each.
A unique characteristic of the Chinese Calendar is the use of the 24 "Jie" and "Qi" or Solar Terms. Each term is about 15 days apart, and in any month there should be a "Jie" and a "Qi". But as there are only 29.5 days in a month, there will be time when a month contained only 1 of the terms. When this happens, the month after will be the additional month added to the year. This is known as double month.
In short, the months keep the calendar in sync with the Moon, and the solar terms keep the calendar in sync with the Sun.