您可以按以下方式扩展DateTime;

public static class DateTimeMethods
{
    public static DateTime StartOfMonth(this DateTime date)
    {
        return new DateTime(date.Year, date.Month, 1, 0, 0, 0);
    }

    public static DateTime EndOfMonth(this DateTime date)
    {
        return date.StartOfMonth().AddMonths(1).AddSeconds(-1);
    }
}

像这样使用它;

DateTime today = DateTime.Now;

DateTime startOfMonth = today.StartOfMonth();
DateTime endOfMonth = today.EndOfMonth();

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

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);

另一种获取结束日期的方法:

    private static DateTime GetMonthEndDate(DateTime date)
    {
        DateTime endDate = date;
        int endDateMonth = endDate.Month;

        while (endDateMonth == endDate.Month)
            endDate = endDate.AddDays(1);

        endDate = endDate.AddDays(-1);

        return endDate;
    }

你可以通过下面的代码找到任何月份的最后一天:

var now = DateTime.Now;
var startOfMonth = new DateTime(now.Year, now.Month, 1);
var DaysInMonth = DateTime.DaysInMonth(now.Year, now.Month);
var lastDay = new DateTime(now.Year, now.Month, DaysInMonth);

我只想在每个月的最后一天发射代码，就是这么简单……

if (DateTime.UtcNow.AddDays(1).Day != 1)
{
    // Tomorrow is not the first...
    return;
}