原文:

写了个小功能，需要用到以周为时间段，于是写了个获取周第一和最后一天的方法，获取月份的第一和最后一天就比较简单了。代码如下：

public class DateTimeTool    {        /// 
        /// 获取指定日期所在周的第一天，星期天为第一天        /// 
        ///         /// 
               public static DateTime GetDateTimeWeekFirstDaySun(DateTime dateTime)        {            DateTime firstWeekDay = DateTime.Now;            try            {                //得到是星期几，然后从当前日期减去相应天数                 int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                int daydiff = (-1) * weeknow;                firstWeekDay = dateTime.AddDays(daydiff);            }            catch { }            return firstWeekDay;        }        /// 
        /// 获取指定日期所在周的第一天，星期一为第一天        /// 
        ///         /// 
               public static DateTime GetDateTimeWeekFirstDayMon(DateTime dateTime)        {            DateTime firstWeekDay = DateTime.Now;            try            {                int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                //星期一为第一天，weeknow等于0时，要向前推6天。                   weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));                int daydiff = (-1) * weeknow;                firstWeekDay = dateTime.AddDays(daydiff);            }            catch { }            return firstWeekDay;        }        /// 
        /// 获取指定日期所在周的最后一天，星期六为最后一天        /// 
        ///         /// 
               public static DateTime GetDateTimeWeekLastDaySat(DateTime dateTime)        {            DateTime lastWeekDay = DateTime.Now;            try            {                int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                int daydiff = (7 - weeknow) - 1;                lastWeekDay = dateTime.AddDays(daydiff);            }catch { }            return lastWeekDay;        }        /// 
        /// 获取指定日期所在周的最后一天，星期天为最后一天        /// 
        ///         /// 
               public static DateTime GetDateTimeWeekLastDaySun(DateTime dateTime)        {            DateTime lastWeekDay = DateTime.Now;            try            {                int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                weeknow = (weeknow == 0 ? 7 : weeknow);                int daydiff = (7 - weeknow);                 lastWeekDay = dateTime.AddDays(daydiff);            }            catch { }            return lastWeekDay;        }        /// 
        /// 获取指定日期的月份第一天        /// 
        ///         /// 
               public static DateTime GetDateTimeMonthFirstDay(DateTime dateTime)        {            if (dateTime == null)            {                dateTime = DateTime.Now;            }            return new DateTime(dateTime.Year, dateTime.Month, 1);        }        /// 
        /// 获取指定月份最后一天        /// 
        ///         /// 
               public static DateTime GetDateTimeMonthLastDay(DateTime dateTime)        {            int day = DateTime.DaysInMonth(dateTime.Year, dateTime.Month);            return new DateTime(dateTime.Year, dateTime.Month, day);        }    }