The Shamsi calendar (also known as the Persian or Jalali calendar) is a solar calendar used in Iran and Afghanistan. It has 12 months, with the first 6 months having 31 days, the next 5 months having 30 days, and the last month having 29 days in common years and 30 days in leap years. The calendar is based on the solar year and is more accurate than the Gregorian calendar for astronomical purposes.