Calculate previous week's start and end date

Question

What is the best way to calculate the previous week\'s start and end date in C#? I.e. today 18 March would result in 9 March (Monday last week) and 15 March (Sunday last we

Answer 1

You can skip the while loop and use

DateTime mondayOfLastWeek = date.AddDays( -(int)date.DayOfWeek - 6 );

This assumes you're using Monday as the first day of the week.

Answer 2

using Fluent DateTime https://github.com/FluentDateTime/FluentDateTime

var dateTime = 1.Weeks().Ago();
var monday = dateTime.Previous(DayOfWeek.Sunday);
var sunday = dateTime.Next(DayOfWeek.Sunday);

Answer 3

The accepted solution is actually not correct.

You have to switch when the week "breaks", i.e. when it considers the week to end or start, and the formula in the accepted solution does not.

This is not so visible on a Monday as week start, but more so if you would consider Thursday the end of the week.

Correct formula is (for a Thursday as ending day):

DateTime thu = date.AddDays(-(int)(date.AddDays(-5).DayOfWeek) -1);

For Monday, the -5 would switch to -2.

Example code to print out

        String s = "";
        DateTime date = new DateTime(2017, 1, 1);
        for (int i = 0; i < 14; i++)
        {
            date = date.AddDays(1);
            DateTime thu = date.AddDays(-(int)(date.AddDays(-5).DayOfWeek) -1);
            DateTime mon = date.AddDays(-(int)(date.AddDays(-2).DayOfWeek) -1);
            s += date.ToString() + " - Thu: " + thu.ToString() + " - Mon: " + mon.ToString() + "\r\n";
        }
        Console.WriteLine(s);

Answer 4

Current accepted answer will fail for Sundays by giving the Monday of the current week instead of last weeks Monday. (code with unit tests output)

DateTime mondayOfLastWeek = date.AddDays( -(int)date.DayOfWeek - 6 );
//Failed to get start of last-week from date-time: 2020-03-01T00:00:00.0000000Z
//  Expected: 2020-02-17 00:00:00.000
//  But was:  2020-02-24 00:00:00.000

The following bit of code addresses this issue:

var dayOfWeek = (int) date.DayOfWeek - 1;
if (dayOfWeek < 0) dayOfWeek = 6;

var thisWeeksMonday = date.AddDays(-dayOfWeek).Date;
var lasWeeksMonday = thisWeeksMonday.AddDays(-7);

which can be reduced to a one-liner should you so desire (I do not recommend using this barely readable code...):

var mondayOfLastWeek  = date.AddDays(-(7 + ((int)date.DayOfWeek - 1 == -1 ? 6 : (int)date.DayOfWeek - 1)));

Answer 5

Using DayOfWeek would be a way of achieving this:

    DateTime date = DateTime.Now.AddDays(-7);
    while (date.DayOfWeek != DayOfWeek.Monday)
    {
        date = date.AddDays(-1);
    }

    DateTime startDate = date;
    DateTime endDate = date.AddDays(7);

Answer 6

DayOfWeek weekStart = DayOfWeek.Monday; // or Sunday, or whenever
DateTime startingDate = DateTime.Today;

while(startingDate.DayOfWeek != weekStart)
    startingDate = startingDate.AddDays(-1);

DateTime previousWeekStart = startingDate.AddDays(-7);
DateTime previousWeekEnd = startingDate.AddDays(-1);

Read: Backtrack one day at a time until we're at the start of this week, and then subtract seven to get to the start of last week.