Calendar Converter. Most people don't realize that the dates recorded by people who lived in the past don't exactly translate to today's dates because they used different calendars. For example, Christopher Columbus said he left on August 3, 1492. However, that date is on the Julian Calendar. Our calendar is Gregorian--which means he actually left on August 12th, 1492. This calendar converter does Gregorian, Hebrew, Islamic, Julian, Mayan, and Persian calendar systems.