Amount of weeks between 2 iso8601 dates

Question

How do you calculate the amount of weeks between 2 ISO8601 dates using only the week and year number?

Input: year1 week1 year2 week2

Output: Amount of weeks according to ISO8601

I can calculate the amount of weeks in a year:

public static int AmountOfWeeksInYearIso8601(this DateTime dateTime)
{
    var year = dateTime.Year;
    var g = Math.Floor((year - 100d) / 400d) - Math.Floor((year - 102d) / 400d);
    var h = Math.Floor((year - 200d) / 400d) - Math.Floor((year - 199d) / 400d);
    var f = 5 * year + 12 - 4 * (Math.Floor(year / 100d) - Math.Floor(year / 400d)) + g + h;

    return f % 28 < 5 ? 53 : 52;
}

Answer 1

Create DateTime values corresponding to the Mondays of the two weeks. See Calculate date from week number for how to do this. Subtract these to get the difference as a TimeSpan. Request its Days property to get the difference as a number of days. Divide by 7 to get the difference as a number of weeks.