Algorithm to add or subtract days from a date?

Question

I\'m trying to write a Date class in an attempt to learn C++.

I\'m trying to find an algorithm to add or subtract days to a date, where Day starts from 1 and Month s

Answer 1

Here's a sketch of a very simple approach. For simplicity of ideas I will assume that d, the number of days to add, is positive. It is easy to extend the below to cases where d is negative.

Either d is less than 365 or d is greater than or equal to 365.

If d is less than 365:

m = 1;
while(d > numberOfDaysInMonth(m, y)) {
    d -= numberOfDaysInMonth(m, y);
    m++;
}
return date with year = y, month = m, day = d;

If d is greater than 365:

while(d >= 365) {
    d -= 365;
    if(isLeapYear(y)) {
        d -= 1;
    }
    y++;
}
// now use the case where d is less than 365

Alternatively, you could express the date in, say, Julian form and then merely add to the Julian form and conver to ymd format.