Get date of first and last day of week knowing week number

Question

I need to get the date of the first and last day of the week knowing the week number.

I get a start date and an end date, representing the first and last day of a s

Answer 1

Subtract 1 year from your original DateTime and call your GetWeekNumber method again with that.

TimeSpan oneYear = dateTime.AddYears(1) - dateTime;
var lastYearWeekNumber = GetWeekNumber(dateTime.Subtract(oneYear));

Answer 2

This is my solution:

public static DateTime FirstDayOfWeek(DateTime date)
{
    DayOfWeek fdow = CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek;
    int offset = fdow - date.DayOfWeek;
    DateTime fdowDate = date.AddDays(offset);
    return fdowDate;
}

public static DateTime LastDayOfWeek(DateTime date)
{
    DateTime ldowDate = FirstDayOfWeek(date).AddDays(6);
    return ldowDate;
}

Answer 3

The method above calculates first date wrong for 1st week of 2016.

Fixed code based on previous answers

 public DateTime FirstDateOfWeek(
        int year,
        int weekOfYear)
    {
        var newYear = new DateTime(year, 1, 1);
        var weekNumber = newYear.GetIso8601WeekOfYear(); 

        DateTime firstWeekDate;

        if (weekNumber != 1)
        {
            var dayNumber = (int) newYear.DayOfWeek;
            firstWeekDate = newYear.AddDays(7 - dayNumber + 1);
        }
        else
        {
            var dayNumber = (int)newYear.DayOfWeek;
            firstWeekDate = newYear.AddDays(-dayNumber + 1);
        }

        if (weekOfYear == 1)
        {
            return firstWeekDate;
        }

        return firstWeekDate.AddDays(7 * (weekOfYear - 1));
    }

Plus test

    [TestCase(2016, 1, 2016, 1, 4)]
    [TestCase(2016, 2, 2016, 1, 11)]
    [TestCase(2016, 52, 2016, 12, 26)]
    [TestCase(2014, 1, 2013, 12, 30)]
    public void FirstDateOfWeek_ShouldReturnCorrectDay(int year, int weekNumber, int correctYear, int correctMonth, int correctDay)
    {
        var helper = new DateHelper();
        var result = helper.FirstDateOfWeek(year, weekNumber);

        Assert.That(result.Year, Is.EqualTo(correctYear));
        Assert.That(result.Month, Is.EqualTo(correctMonth));
        Assert.That(result.Day, Is.EqualTo(correctDay));
    }

Answer 4

Tim Schmelter's method didn't work for Swedish (sv-SE) culture where weeks start on Mondays. This has been tested with en-US and sv-SE.

    public static DateTime GetFirstDateOfWeek(int year, int weekOfYear, CultureInfo cultureInfo)
    {
        DateTime jan1 = new DateTime(year, 1, 1);
        int daysOffset = (int)cultureInfo.DateTimeFormat.FirstDayOfWeek - (int)jan1.DayOfWeek;
        DateTime firstWeekDay = jan1.AddDays(daysOffset);
        int firstWeek = cultureInfo.Calendar.GetWeekOfYear(jan1, cultureInfo.DateTimeFormat.CalendarWeekRule, cultureInfo.DateTimeFormat.FirstDayOfWeek);
        if (firstWeek == 1)
        {
            weekOfYear -= 1;
        }
        return firstWeekDay.AddDays(weekOfYear * 7);
    }

Answer 5

    private static DateTime FirstDateOfWeek(int year, int weekOfYear)
    {
        var firstDate = new DateTime(year, 1, 4);
        //first thursday of the week defines the first week (https://en.wikipedia.org/wiki/ISO_8601)
        //Wiki: the 4th of january is always in the first week
        while (firstDate.DayOfWeek != DayOfWeek.Monday)
            firstDate = firstDate.AddDays(-1);

        return firstDate.AddDays((weekOfYear - 1)*7);
    }

Easyer and more efficient solution!

Answer 6

Since there are 7 days per week, you will know that Week 10 is the same as days 71 - 77. You know the number of days per month (don't forget leap year). Construct the exact dates and then, you can get information about those days.