Leap year birthdays can be a source of legal issues for people born on February 29th (often called “leapers”), particularly with regard to medical or government records. Some computer systems do not accept February 29th as a valid date, or alternatively do not take into account leap years and print invalid expiration dates (such as February 29, 2015, which did not exist) on driver’s licenses, insurance policies, and other documents. Because of the potential issues related to having a birthday that is only officially recognized every four years, there are recorded instances of physicians altering birth certificates so that the birth date would not fall on February 29th — an illegal practice.