计算两个日期之间第一天有多少个星期一

Question

Is it possible to calculate how many Monday the 1st are between two dates?

I wrote two functions to find how many days are between two dates, using C++:

int rdn(int y, int m, int d) {
    if (m < 3) { y--, m += 12; };
    return 365*y + y/4 - y/100 + y/400 + (153*m - 457)/5 + d - 306;
}

int days = rdn(2015, 01, 01) - rdn(2015, 12, 31);

And using Python:

def days_between(d1, d2):
    d1 = datetime.strptime(d1, "%Y-%m-%d")
    d2 = datetime.strptime(d2, "%Y-%m-%d")
    return abs((d2 - d1).days)

print(days_between('2015-01-01', '2015-12-31'))

But I don't understand how to update my function to calculate Monday the 1st.

Answer 1

This algorithm is a loop that examines the weekday of the 1st of each month from the start date (inclusive) to the end date (inclusive), and counts the ones which fall on a Monday:

import datetime

def num_monday_the_first_between(start, end):
    y, m, d = start.year, start.month, 1
    count = 0
    while (y, m, d) <= (end.year, end.month, end.day):
        if datetime.date(y, m, d).isoweekday() == 1:
            count += 1
        m += 1
        if m == 13:
            m = 1
            y += 1
    return count


print(num_monday_the_first_between(datetime.date(2015,1,1), datetime.date(2015,12,31)))
# Result: 1 (June)