Thread safe use of java.util.SimpleDateFormat

When playing with [Gatling](www.gatling-tool.org), a load testing tool, I often choose the Computer-Database sample application as a victim. When massively sending POST requests to create new computers in the application, I randomly face some 400 errors. 

As described in the [Oracle Javadoc](http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html), SimpleDateFormat is not threadsafe. 
This is the root of my random errors.

Nicolas