DateFormat shouldn't be static

[ZiglioUK]

Or any Time related objects (with the old Time classes, before Java 8)
They're not threadsafe

    private static DateFormat df = new SimpleDateFormat("yyyy MM dd HH mm ss.SSS");

    private static Date[] leapDates = new Date[17];

https://code.google.com/p/gogps/source/browse/src/main/java/org/gogpsproject/Time.java