In East Asian cultures, namely Chinese, Korean, Japanese, and Vietnamese, a person's age differs from the Western calendar as it aligns with the lunar calendar. Traditionally, the Chinese consider the time from conception to birth to be one year and believe this time is part of a person's age. This age is used in various ways such as predicting a baby's gender and discovering their Chinese zodiac or birth sign.
In this East Asian concept and practice, newborns start at one year old instead of zero. Thus, in his second year, the person is considered two years old, in the third year, three years old, so on and so forth. Therefore, to find out one's lunar age, the simplest method is typically to add one year to his Western age.
Because the Lunar New Year typically falls between late January and late February, being born before or after the date of that year will determine which year a person is born. For instance, if one was born on February 1, 1980, and the Lunar New Year was February 20th of that year, the person's lunar birth year would actually be 1979. Thus, a good method is to find the date when the Lunar New Year fell on the year one was born, subtract that from the present year, and then add one.