from datetime import date, timedelta

class DateWithCalendar(object):
    def __init__(self, calendar_class, date):
        self.calendar = calendar_class
        self.date = date

    def convert_to(self, calendar):
        return calendar.from_date(self.date)

    def __eq__(self, other):
        return self.calendar == other.calendar and self.date == other.date

class Calendar(object):
    def from_date(self, date):
        return DateWithCalendar(self.__class__, date)

class ProlepticGregorianCalendar(Calendar):
    def date(self, year, month, day):
        d = date(year, month, day)
        return self.from_date(d)

class JulianCalendar(Calendar):
    @staticmethod
    def is_julian_leap_year(y):
        return (y % 4) == 0

    def date(self, year, month, day):
        if day == 29 and month == 2 and self.is_julian_leap_year(year):
            d = date(year, month, 28)
            d = d + timedelta(days=11)
        else:
            d = date(year, month, day)
            d = d + timedelta(days=10)
        return self.from_date(d)


from datetime import date, timedelta

class DateWithCalendar(object):
    def __init__(self, calendar_class, date):
        self.calendar = calendar_class
        self.date = date

    def convert_to(self, calendar):
        return calendar.from_date(self.date)

    def __eq__(self, other):
        return self.calendar == other.calendar and self.date == other.date

class Calendar(object):
    def from_date(self, date):
        return DateWithCalendar(self.__class__, date)

class ProlepticGregorianCalendar(Calendar):
    def date(self, year, month, day):
        d = date(year, month, day)
        return self.from_date(d)

class JulianCalendar(Calendar):
    def date(self, year, month, day):
        d = date(year, month, day)
        d = d + timedelta(days=10)
        return self.from_date(d)


--- 
+++ 
@@ -21,8 +21,16 @@
         return self.from_date(d)
 
 class JulianCalendar(Calendar):
+    @staticmethod
+    def is_julian_leap_year(y):
+        return (y % 4) == 0
+
     def date(self, year, month, day):
-        d = date(year, month, day)
-        d = d + timedelta(days=10)
+        if day == 29 and month == 2 and self.is_julian_leap_year(year):
+            d = date(year, month, 28)
+            d = d + timedelta(days=11)
+        else:
+            d = date(year, month, day)
+            d = d + timedelta(days=10)
         return self.from_date(d)
 
Handle julian leap days separately.
