格式：DateTime.Compare(datetime1, datetime2)
参数为时间格式，为第一个参数比较第二个参数，返回小于0的值，等于0或大于0的值。
实例:
1 string st1 = "12:13";
2 string st2 = "14:13";
3 DateTime t1 = Convert.ToDateTime(st1);
4 Console.WriteLine(t1); //20186/6 下午 12:12:00
5 DateTime t2 = Convert.ToDateTime(st2);
6 Console.WriteLine(t2); //20186/6 下午 02:14:00
7 DateTime t3 = DateTime.Now;
8 Console.WriteLine(t3);
9 if (DateTime.Compare(t1, t2) > 0) Console.WriteLine(st1+">"+st2);
10 else if (DateTime.Compare(t1, t2) == 0) Console.WriteLine(st1+"="+st2);
11 else Console.WriteLine(st1+"<"+st2);
12
13 if (DateTime.Compare(t1, t3) > 0) Console.WriteLine("
"+st1 + ">" + t3.ToString());
14 else if (DateTime.Compare(t1, t3) == 0) Console.WriteLine("
" + st1 + "=" + t3.ToString());
15 else Console.WriteLine("
" + st1 + "<" + t3.ToString());