C#后台的一些常用的公用方法--时间计算篇

1.获取当前周

//获取当前日期是本年度的第几周
int thisWeek = WeekOfYear(DateTime.Now.AddDays(1).ToString());

//获得当前周
        public static int WeekOfYear(string date)
        {
            DateTime curDay = Convert.ToDateTime(date);
            int firstdayofweek = Convert.ToInt32(Convert.ToDateTime(curDay.Year.ToString() + "- " + "1-1 ").DayOfWeek);
            int days = curDay.DayOfYear;
            int daysOutOneWeek = days - (7 - firstdayofweek);
            if (daysOutOneWeek <= 0)
            {
                return 1;
            }
            else
            {
                int weeks = daysOutOneWeek / 7;
                if (daysOutOneWeek % 7 != 0)
                    weeks++;
                return weeks + 1;
            }
        }

2.获取指定周数的第一天的日期和最后一天的日期（默认周日为一周的第一天）

//指定周的第一天和最后一天的日期
GetDaysOfWeeks(int.Parse(thisYear), thisWeek, out DateTime firstDay, out DateTime lastDay);

/// <summary>
        /// 获取指定周数的开始日期和结束日期，开始日期为周日
        /// </summary>
        /// <param name="year">年份</param>
        /// <param name="index">周数</param>
        /// <param name="first">当此方法返回时，则包含参数 year 和 index 指定的周的开始日期的 System.DateTime 值；如果失败，则为 System.DateTime.MinValue。</param>
        /// <param name="last">当此方法返回时，则包含参数 year 和 index 指定的周的结束日期的 System.DateTime 值；如果失败，则为 System.DateTime.MinValue。</param>
        /// <returns></returns>
        public static bool GetDaysOfWeeks(int year, int index, out DateTime first, out DateTime last)
        {
            first = DateTime.MinValue;
            last = DateTime.MinValue;
            if (year < 1700 || year > 9999)
            {
                //"年份超限"
                return false;
            }
            if (index < 1 || index > 53)
            {
                //"周数错误"
                return false;
            }
            DateTime startDay = new DateTime(year, 1, 1);  //该年第一天
            DateTime endDay = new DateTime(year + 1, 1, 1).AddMilliseconds(-1);
            int dayOfWeek = 0;
            if (Convert.ToInt32(startDay.DayOfWeek.ToString("d")) > 0)
                dayOfWeek = Convert.ToInt32(startDay.DayOfWeek.ToString("d"));  //该年第一天为星期几
            if (dayOfWeek == 7) { dayOfWeek = 0; }
            if (index == 1)
            {
                first = startDay;
                if (dayOfWeek == 6)
                {
                    last = first;
                }
                else
                {
                    last = startDay.AddDays((6 - dayOfWeek));
                }
            }
            else
            {
                first = startDay.AddDays((7 - dayOfWeek) + (index - 2) * 7); //index周的起始日期
                last = first.AddDays(6);
                if (last > endDay)
                {
                    last = endDay;
                }
            }
            if (first > endDay)  //startDayOfWeeks不在该年范围内
            {
                //"输入周数大于本年最大周数";
                return false;
            }
            return true;
        }

3.获取指定年月的第一天和最后一天的日期

//指定年月的第一天最后一天
string firstDayOfThisMonth = GetCurMonthFirstDay(DateTime.Now.Year.ToString(), DateTime.Now.Month.ToString()).ToString("yyyyMMdd");
                string lastDayOfThisMonth = GetCurMonthLastDay(DateTime.Now.Year.ToString(), DateTime.Now.Month.ToString()).ToString("yyyyMMdd");

//获取指定年月的第一天
        public static DateTime GetCurMonthFirstDay(string year, string mon)
        {
            DateTime AssemblDate = Convert.ToDateTime(year + "-" + mon + "-" + "01");  // 组装当前指定月份
            return AssemblDate.AddDays(1 - AssemblDate.Day);  // 返回指定当前月份的第一天
        }


        //获取指定年月的最后一天
        public static DateTime GetCurMonthLastDay(string year, string mon)
        {
            DateTime AssemblDate = Convert.ToDateTime(year + "-" + mon + "-" + "01");  // 组装当前指定月份
            return AssemblDate.AddDays(1 - AssemblDate.Day).AddMonths(1).AddDays(-1);  // 返回指定当前月份的最后一天
        }