根据当前的日期获取到周起始，月起始，年起始

一、根据当前的日期获取到周起始和截止

/// <summary>
/// 得到本周第一天(以星期天为第一天)
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public DateTime GetWeekFirstDaySun(DateTime datetime)
{
    //星期天为第一天
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);
    int daydiff = (-1) * weeknow;

    //本周第一天
    string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(FirstDay);
}

/// <summary>
/// 得到本周第一天(以星期一为第一天)
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public DateTime GetWeekFirstDayMon(DateTime datetime)
{
    //星期一为第一天
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);

    //因为是以星期一为第一天，所以要判断weeknow等于0时，要向前推6天。
    weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));
    int daydiff = (-1) * weeknow;

    //本周第一天
    string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(FirstDay);
}

/// <summary>
/// 得到本周最后一天(以星期六为最后一天)
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public DateTime GetWeekLastDaySat(DateTime datetime)
{
    //星期六为最后一天
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);
    int daydiff = (7 - weeknow) - 1;

    //本周最后一天
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(LastDay);
}

/// <summary>
/// 得到本周最后一天(以星期天为最后一天)
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public DateTime GetWeekLastDaySun(DateTime datetime)
{
    //星期天为最后一天
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);
    weeknow = (weeknow == 0 ? 7 : weeknow);
    int daydiff = (7 - weeknow);

    //本周最后一天
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(LastDay);
}

二、根据当前的日期获取到月起始和截止

int year = DateTime.Now.Year;
int month = DateTime.Now.Month;

DateTime firstDayOfThisMonth = new DateTime(year, month, 1);
DateTime lastDayOfThisMonth = new DateTime(year, month, DateTime.DaysInMonth(year, month));

     SQL语句实现

DECLARE @NOW DATETIME
DECLARE @FistDayOfThisMonth DATETIME  
DECLARE @LastDayOfThisMonth DATETIME  

SET @NOW = GETDATE()
SET @FistDayOfThisMonth = @NOW - DAY(@NOW) + 1  
SET @LastDayOfThisMonth = DATEADD(MONTH, 1, @NOW - DAY(@NOW) + 1) - 1

SELECT @FistDayOfThisMonth
SELECT @LastDayOfThisMonth

三、根据当前的日期获取到年起始和截止

int year = DateTime.Now.Year;
int month = DateTime.Now.Month;

DateTime firstDayOfThisMonth = new DateTime(year, 1, 1);
DateTime lastDayOfThisMonth = new DateTime(year, 12, DateTime.DaysInMonth(year, 12));