C# DateTime应用

1、获取当月第一天0时0分0秒

DateTime.Now.AddDays(1 - DateTime.Now.Day);

Convet.ToDateTime(DateTime.Now.ToString("yyyy-MM-01"));

2、获取当月最后一天23时59分59秒：

DateTime.Now.AddDays(1 - DateTime.Now.Day).AddMonths(1).AddSeconds(-1);

Convet.ToDateTime(DateTime.Now.AddMonths(1).ToString("yyyy-MM-01")).AddSeconds(-1);

3、获取当月后一月第一天0时0分0秒

DateTime.Now.AddDays(1 - DateTime.Now.Day).AddMonths(1)

Convet.ToDateTime(DateTime.Now.AddMonths(1).ToString("yyyy-MM-01"));

4、获取当月前一月第一天0时0分0秒

DateTime.Now.AddDays(1 - DateTime.Now.Day).AddMonths(-1)

Convet.ToDateTime(DateTime.Now.AddMonths(-1).ToString("yyyy-MM-01"));

5、判断时间是否在指定月份内

public bool HasDomain(DateTime testTime,DateTime domain){

    DateTime startTime = domain.AddDays(1 - domain.Day);//当前月份第一天2019/12/1 00：00：00
    DateTime endTime = domain.AddDays(1 - domain.Day).AddMonths(1);//当前月份后一月的第一天2020/1/1 00：00：00
    if(testTime>=startTime&&testTime<endTime){
        return true;
    }else{
        return false;   
    }
}

6、获取指定日期是一年中的第几周

 public int WeekOfYear(DateTime dtime){
    int weeknum = 0;
    DateTime tmpdate = DateTime.Parse(dtime.Year.ToString() + "-1" + "-1");
    DayOfWeek firstweek = tmpdate.DayOfWeek;
    int i = dtime.DayOfYear - 1 + (int)firstweek;
    weeknum = i / 7;
    if (i > 0) weeknum++;
    return weeknum;
}

7、获取指定日期的星期一

public DateTime GetMonday(DateTime dateTime){
    int monDiff=dateTime.DayOfWeek-DayOfWeek.Monday;
    monDiff = monDiff == -1 ? 6 : monDiff;
    DateTime monTime = dTimedateTimeAddDays(-monDiff);
    return monTime;
}

8、获取指定日期的星期天

public DateTime GetSunday(DateTime dateTime){
    int sunDiff = dTime.DayOfWeek - DayOfWeek.Sunday;
    sunDiff = sunDiff !=0 ? 7-sunDiff : sunDiff;
    DateTime sunTime = dTime.AddDays(sunDiff);
}