Convert.ToDateTime()时间格式

Convert.ToDateTime()时间后面怎么加上23:59:59.000

最近做一个查询，需要精确到时分秒，但是我去前台选定的时间：

DateTime jssj = Convert.ToDateTime(this.jssj.Value);

获取的结束时间是：2019-03-31 00:00:00.000

怎么把 2019-03-31 00:00:00.000 后面的 00:00:00.000 改成23:59:59.000
源代码如下：

DateTime kssj = Convert.ToDateTime(this.kssj.Value);
DateTime jssj = Convert.ToDateTime(this.jssj.Value);
DateTime kssjjy = kssj.AddDays(-1);
DateTime jssjjy = jssj.AddDays(1);

DateTime dtime = Convert.ToDateTime(this.jssj.Value);
DateTime kssj = Convert.ToDateTime(this.kssj.Value);
DateTime jssj = new DateTime(dtime.Year,dtime.Month,dtime.Day, 23,59,59);
DateTime kssjjy = new DateTime(kssj.Year, kssj.Month, kssj.Day, 23, 59, 59).AddDays(-1);
DateTime jssjjy = new DateTime(jssj.Year, jssj.Month, jssj.Day, 00, 00, 00).AddDays(1);
string HName = this.mc.SelectedValue.ToString();

string time=jssj.ToShortDateString().ToString()+" 23:59:59.000";