[internship experience] add your own implementation method to the date tool class

I feel that recently java The date format conversion and date processing in , Make a note of

First , First define the time format constant in the class ：

	/** *  The format of time  */
    public static final String TIME_WITH_SEPARATOR = "yyyy-MM-dd HH:mm:ss";
    public static final String TIME = "yyyyMMddHHmmss";