Language.getDateFormat() is not thread safe

Language.getDateFormat() return a SimpleDateFormat instance variable. Since language instance can be cached in the system and SimpleDateFormat is not thread safe, this can cause unexpected exception when server is under high load (see https://stackoverflow.com/questions/18383251/strange-arrayindexoutofboundsexception-for-java-simpledateformat). We did come across this issue before when there are many scheduler process fire at the same time.