日期和时间

 var nowtime = DateTime.Now;   // 2023/6/25 11:18:241

var end = DateTime.Now.EndTimeOfDay();   //  2023 6/25 23:59:59   

----返回日期最后一刻，精确到23:59:59.999

var start= DateTime.Now.StartTimeOfDay(); //   2023/6/25 0:00:00

---返回日期开始一刻,精确到0:0:0.0

  var monthst= DateTime.Now.StartTimeOfMonth();  //  2023/6/1 0:00:00

  var monthed = DateTime.Now.EndTimeOfMonth();  //   2023/6/30 23:59:59

 帮助类下载链接DateHelper.cs ：   https://www.aliyundrive.com/s/dWKfUjUYurd