Fix year detection at year turns

calendarweek/calendarweek.py

@@ -64,7 +64,12 @@ class CalendarWeek:
         """ Get the calendar week by a date object (the week this date is in). """
 
         week = int(when.strftime("%V"))
-        year = when.year + 1 if when.month == 12 and week == 1 else when.year
+        year = when.year
+
+        if when.month == 12 and week == 1:
+            year += 1
+        elif when.month == 1 and (week == 52 or week == 53):
+            year -= 1
 
         return cls(year=year, week=week)