Sonarr/NzbDrone.Common/DateTimeExtensions.cs

using System;
using System.Globalization;

namespace NzbDrone.Common
{
    public static class DateTimeExtensions
    {
        public static DateTime GetFirstDayOfWeek(this DateTime dayInWeek)
        {
            var defaultCultureInfo = CultureInfo.CurrentCulture;
            return GetFirstDayOfWeek(dayInWeek, defaultCultureInfo);
        }

        public static DateTime GetFirstDayOfWeek(this DateTime dayInWeek, CultureInfo cultureInfo)
        {
            DayOfWeek firstDay = cultureInfo.DateTimeFormat.FirstDayOfWeek;
            DateTime firstDayInWeek = dayInWeek.Date;
            while (firstDayInWeek.DayOfWeek != firstDay)
                firstDayInWeek = firstDayInWeek.AddDays(-1);

            return firstDayInWeek;
        }
    }
}