说明：  jsp中的时间操作 经常看见jsp版里有人问时间操作的问题，这些问题一般包括：取当前时间，把一个指定的字符串时间转化成时间类型，求两个时间之间的天数，求一段时间以前的时间，求一段时间以后的时间，在这里就把这些问题汇总一下。
(jsp the time jsp operation often see someone asked version operation of the time, These problems generally include : from the current time, a time designated by the string into time type, for the time between the two days, for a period of time before the time for a period of time after time, put in here about these issues matrix.)