problem019.py

#!/usr/bin/python3
#
# Project Euler (projecteuler.net) - Problem 19
# You are given the following information, but you may prefer to do some 
# research for yourself.
#
# 1 Jan 1900 was a Monday.
# Thirty days has September,
# April, June and November.
# All the rest have thirty-one,
# Saving February alone,
# Which has twenty-eight, rain or shine.
# And on leap years, twenty-nine.
# A leap year occurs on any year evenly divisible by 4, but not on a century 
# unless it is divisible by 400.
# How many Sundays fell on the first of the month during the twentieth century
#  (1 Jan 1901 to 31 Dec 2000)?

import datetime
from datetime import timedelta
cur = datetime.date( 1901, 1, 1 )
cur = cur + timedelta( days=6-cur.weekday() )
sundays = 0
while cur < datetime.date( 2000, 12, 31 ):
    cur += timedelta( days=7 )
    if cur.day == 1: sundays += 1
print( sundays )