这个月的最后一天你会得到这样的结果，返回31:

DateTime.DaysInMonth(1980, 08);

从DateTimePicker:

第一次约会:

DateTime first_date = new DateTime(DateTimePicker.Value.Year, DateTimePicker.Value.Month, 1);

最后的日期:

DateTime last_date = new DateTime(DateTimePicker.Value.Year, DateTimePicker.Value.Month, DateTime.DaysInMonth(DateTimePicker.Value.Year, DateTimePicker.Value.Month));

要获得特定日历中一个月的最后一天-并在扩展方法中-:

public static int DaysInMonthBy(this DateTime src, Calendar calendar)
{
    var year = calendar.GetYear(src);                   // year of src in your calendar
    var month = calendar.GetMonth(src);                 // month of src in your calendar
    var lastDay = calendar.GetDaysInMonth(year, month); // days in month means last day of that month in your calendar
    return lastDay;
}

从下个月的第一天减去一天:

DateTime lastDay = new DateTime(MyDate.Year,MyDate.Month+1,1).AddDays(-1);

此外，如果你也需要它在12月工作:

DateTime lastDay = new DateTime(MyDate.Year,MyDate.Month,1).AddMonths(1).AddDays(-1);

var lastDayOfMonth = DateTime.DaysInMonth(date.Year, date.Month);

例如2021年5月7日

DateTime.Now.Day;

结果:5

DateTime.Now.AddMonths（+1）.AddDays（-DateTime.Now.Day）.ToString（“yyyy-MM-dd”）;

结果:2021/07/31