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

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)));