Technically, Chinese people have two birthdays (生日 shēngrì)! Thanks to the continued use of the traditional lunar calendar, every day has two dates: lunar and official.
Officially, China uses the international standard calendar (Gregorian) so that January 1st is the same date all around the world. Even though the Chinese government started using the international standard calendar in 1949, the traditional lunar calendar is still used for selecting lucky dates for important events like weddings, funerals, and business openings.
The Chinese lunar calendar (阴历 yīnlì) is similar to traditional calendars used throughout Asia and in Jewish and Muslim cultures. Each month starts on the night of a new moon and lasts for one full cycle of the moon (29.5 days). The year starts with the Spring Festival (Chinese New Year) which is the 1st day of the 1st month and ranges from January 21st to February 20th on the international standard calendar. There are still 12 months in each year except for every few years when an extra month is added to maintain alignment with the international calendar. The next 13th month will be added in 2020.
While most people know which month and day of the lunar calendar they were born, their identification card and official documents all record their standard calendar birthdate. So that’s the birthday that many people celebrate in China, especially young people. Although many people of the older generations, in rural areas, or those who want to preserve Chinese traditions, celebrate their lunar calendar birthday.
Another traditional difference with birthdays in China is your age (年龄 niánlíng). When a child is born, they have already spent approximately 9 months growing so they are considered to be 1 year old when they are born. However, as with calendars, official age in China is counted the same way as in other countries.