SimpleDateFormat is not thread-safe

The `SimpleDateFormat` class is not thread-safe. 
Here, I patched it the dummiest way by creating a new instance every time.  

The better solution I can see are:
* adding a dependency to the Joda/ThreeTen
* adding a dependency to commons-lang for `FastDateFormat`
* have the  `SimpleDateFormat` class in a `ThreadLocal` - but that may be prone to memory leaks

What do you think?