find the bug part one

find-the-bug1.py

import calendar
import sys

def ConvertDays(days):
        year = 1980
        while days > 365:
                if calendar.isleap(year):
                        if days > 366:
                                days -= 366
                                year += 1
                else:
                        days -= 365
                        year += 1
        return year

if len(sys.argv) < 2:
        print("Specify number of days since 1980")
        sys.exit(1)

print("Year: %i" % ConvertDays(int(sys.argv[1])))