How accurate was the Julian calendar?

   The Julian calendar was off by eleven minutes and fourteen seconds each year. That amount of time might not sound like much, but over four centuries it added up to just over three days. One consequence of this change was that the vernal equinox (the first day of spring) slipped back from March 25 to March 21. By the mid-1500s the calendar was another ten days ahead of the Earth's natural yearly cycle.