The time that Chinese New Year happens is based off of the lunisolar calendar. That means that instead of basing the calendar off the sun, the phases of the moon are taken into account when determing the year as well as the solar year. The Gregorian Calendar (the one that is commonly used in American and elsewhere) is not taken into account when determing the time of the New Year. Therefore, the beginning of the Gregorian New Year and the beginning of the Lunar New Year will differ by one to two months.
This causes a problem when attempting to calculate a lunar date based on a Gregorian date, that being some will not take into consideration the difference and will assume the beginning of the Gregorian year is the same as the beginning of the Lunar year. This leads to the assumption that those born in the time from the Gregorian New Year to the Lunar New Year will be counted in that Lunar Year. Careful examination will show that this is not the case. Therefore, it is a good idea to check the dates for Lunar New Years before attempting to convert dates from the Gregorian to the Lunar.