1970年1月1日到现在的毫秒数

Long time = new Date().getTime();

/**
* Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT
* represented by this <tt>Date</tt> object.
*
* @return  the number of milliseconds since January 1, 1970, 00:00:00 GMT
*          represented by this date.
*/
public long getTime() {
    return getTimeImpl();
}