The old style is the Julian calendar used by the Orthodox Church for many liturgical dates. The new style is the civil Gregorian calendar, which is used for the modern secular date. In the 20th and 21st centuries, the difference between the old and new styles is 13 days. Therefore, September 1 in the old style falls on September 14 in the new style. After 2100, the difference will increase, and the site automatically takes this into account.