cppDateDiff

#include <iostream>

bool leapYear(int year)
{
    return (year % 400 == 0) or ((year % 100 != 0) and (year % 4 == 0));
}

int daysTillMonth(int month, int year)
{
    const int monthInYear{ 12 };
    const int daysInMonth[monthInYear]{ 31,28 + leapYear(year),31,30,31,30,31,31,30,31,30,31 };
    int days{ 0 };

    --month;
    for (int curMonth{ 0 }; curMonth < month; days += daysInMonth[curMonth++]);

    return days;
}

int dateDiff(int dayBegin, int monthBegin, int yearBegin, int dayEnd, int monthEnd, int yearEnd)
{
    const int daysInYear{ 365 };
    int days{ (yearEnd - yearBegin) * daysInYear };

    for (int year{ yearBegin }; year <= yearEnd; days += leapYear(year++));

    days -= (daysTillMonth(monthBegin, yearBegin) + dayBegin);
    days += (daysTillMonth(monthEnd, yearEnd) + dayEnd);

    return days;
}

int main()
{
    std::cout << dateDiff(15, 5, 1800, 15, 5, 2020) << '\n';
}