How to get 1st Week Date from Week No of the Current Year

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Hi !

I am able to get Week No. of the Current Date (Code):

DateTime dt1 = Convert.ToDateTime(DateTime.Now.ToString("MM/dd/yyyy"));
int yyyy = dt1.Year; int MM = dt1.Month; int dd = dt1.Day;
        System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
        Int32 weekNo = ci.Calendar.GetWeekOfYear(new DateTime(yyyy, MM, dd), ci.DateTimeFormat.CalendarWeekRule, ci.DateTimeFormat.FirstDayOfWeek );

Now I want to get 1st Date from a Given Week No of the given Year.
Please suggest ...

Posted 14-Dec-12 21:26pm

LebneizTech

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arya.anu 15-Dec-12 6:03am

The code correctly puts the start of week 1, 2009 at 29-12-2008. The CalendarWeekRule probably should be a parameter.

Note that the weekNum should be >= 1

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arya.anu · Answer 1 · 2012-12-15T00:02:00

C#

static DateTime FirstDateOfWeek(int year, int weekNum, CalendarWeekRule rule)
{
    Debug.Assert(weekNum >= 1);

    DateTime jan1 = new DateTime(year, 1, 1);

    int daysOffset = DayOfWeek.Monday - jan1.DayOfWeek;
    DateTime firstMonday = jan1.AddDays(daysOffset);
    Debug.Assert(firstMonday.DayOfWeek == DayOfWeek.Monday);

    var cal = CultureInfo.CurrentCulture.Calendar;
    int firstWeek = cal.GetWeekOfYear(firstMonday, rule, DayOfWeek.Monday);

    if (firstWeek <= 1)
    {
        weekNum -= 1;
    }

    DateTime result = firstMonday.AddDays(weekNum * 7);

Shahin Khorshidnia · Answer 2 · 2012-12-15T03:06:00

Hi, I wrote it by hand but hope it'll help :)

C#

public static DateTime GetYearWeekFirstDay(int year, int week)
{            
    var days = 7 * (week - 1) + 1;

    if (days > 366)
        throw new InvalidDataException("Invalid week numbers!");

    var date = new DateTime(year, 1, 1);

    if (date.DayOfWeek != DayOfWeek.Sunday)
        date = date.AddDays(7 - (int)(date.DayOfWeek));

    var count = 1;

    while (count != days)
    {
        count++;
        date = date.AddDays(1);
    }

    return date;
}

How to get 1st Week Date from Week No of the Current Year

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