Original: As the Chinese New Year starts in late January to mid-February, the Chinese year dates from January 1 until that day in the new Gregorian year remain unchanged from the previous Gregorian year.
Simplified: Because the Chinese New Year starts in late January or early February, the Chinese year begins several weeks after the it begins in the Gregorian calendar. If a person is born in early January, their Chinese birth-year is the last Gregorian year, not the current one.