Count number of including weeks between 2 dates

Question

I need to get the number of weeks between 2 dates.
A week for me is monday to sunday.
So if the first date is on a saturday than this week should be included.
if the second date is on a monday than this week should be included.

What is the most efficient way to do this ?

example :

startdate   enddate      nbr of weeks
17/09/2016  26/09/2016   3 weeks
17/09/2016  25/09/2016   2 weeks
19/09/2016  26/09/2016   2 weeks
12/09/2016  25/09/2016   2 weeks

I found much answers for this, like this one for example how to calculate number of weeks given 2 dates? but they all end up with dividing the days with 7 and that does not gives the result I need.

@Roy_Dorsthorst: I don't see how that would be part of the solution. That's just formatting - it has nothing to do with calculating the difference between dates. — Jon Skeet, Sep 30 '16 at 09:50

Jon Skeet · Accepted Answer · 2016-09-30T09:53:23.347

The simplest way is probably to write a method to get the start of a week. Then you can subtract one date from another, divide the number of days by 7 and add 1 (to make it inclusive).

Personally I'd use Noda Time for all of this, but using DateTime:

// Always uses Monday-to-Sunday weeks
public static DateTime GetStartOfWeek(DateTime input)
{
    // Using +6 here leaves Monday as 0, Tuesday as 1 etc.
    int dayOfWeek = (((int) input.DayOfWeek) + 6) % 7;
    return input.Date.AddDays(-dayOfWeek);
}

public static int GetWeeks(DateTime start, DateTime end)
{
    start = GetStartOfWeek(start);
    end = GetStartOfWeek(end);
    int days = (int) (end - start).TotalDays;
    return (days / 7) + 1; // Adding 1 to be inclusive
}

Complete example:

using System;

class Program
{
    static void Main (string[] args)
    {
        ShowWeeks(new DateTime(2016, 9, 17), new DateTime(2016, 9, 26));
        ShowWeeks(new DateTime(2016, 9, 17), new DateTime(2016, 9, 25));
        ShowWeeks(new DateTime(2016, 9, 19), new DateTime(2016, 9, 26));
        ShowWeeks(new DateTime(2016, 9, 12), new DateTime(2016, 9, 25));
    }

    static void ShowWeeks(DateTime start, DateTime end)
    {
        int weeks = GetWeeks(start, end);
        Console.WriteLine($"{start:d} {end:d} {weeks}");
    }

    // Always uses Monday-to-Sunday weeks
    public static DateTime GetStartOfWeek(DateTime input)
    {
        // Using +6 here leaves Monday as 0, Tuesday as 1 etc.
        int dayOfWeek = (((int) input.DayOfWeek) + 6) % 7;
        return input.Date.AddDays(-dayOfWeek);
    }

    public static int GetWeeks(DateTime start, DateTime end)
    {
        start = GetStartOfWeek(start);
        end = GetStartOfWeek(end);
        int days = (int) (end - start).TotalDays;
        return (days / 7) + 1; // Adding 1 to be inclusive
    }
}

Output (in my UK locale):

17/09/2016 26/09/2016 3
17/09/2016 25/09/2016 2
19/09/2016 26/09/2016 2
12/09/2016 25/09/2016 2

Either `GetEndOfWeek` is a typo or you omitted a method. – Abion47 Sep 30 '16 at 09:52 — Abion47, Sep 30 '16 at 09:52
@Abion47: Typo - fixed now. – Jon Skeet Sep 30 '16 at 09:53 — Jon Skeet, Sep 30 '16 at 09:53

Slai · Answer 2 · 2016-09-30T14:49:10.433

5

A bit "simplified", because the Monday is needed just for the start date:

static int weeks(DateTime d1, DateTime d2) { 
    var daysSinceMonday = ((int)d1.DayOfWeek + 6) % 7;
    return ((d2 - d1).Days + daysSinceMonday) / 7 + 1; 
}

edited Sep 30 '16 at 14:49

answered Sep 30 '16 at 13:09

Slai

22,144
5
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matthijsb · Answer 3 · 2016-09-30T10:33:05.827

See my method below, I "strechted" the weeks to monday untill sunday, and then calculated the total days / 7

    public static void Main(string[] args)
    {
        Console.WriteLine(CalculateWeeks(new DateTime(2016, 9, 17), new DateTime(2016, 9, 26)));
        Console.WriteLine(CalculateWeeks(new DateTime(2016, 9, 17), new DateTime(2016, 9, 25)));
        Console.WriteLine(CalculateWeeks(new DateTime(2016, 9, 19), new DateTime(2016, 9, 26)));
        Console.WriteLine(CalculateWeeks(new DateTime(2016, 9, 12), new DateTime(2016, 9, 25)));

    }

    public static double CalculateWeeks(DateTime from, DateTime to)
    {
        if (to.DayOfWeek != DayOfWeek.Sunday)
            to = to.Add(new TimeSpan(7- (int) to.DayOfWeek, 0, 0, 0)).Date;
        return Math.Ceiling((to - from.Subtract(new TimeSpan((int)from.DayOfWeek - 1, 0, 0, 0)).Date).TotalDays / 7);
    }

Count number of including weeks between 2 dates

3 Answers3