DateFormat is a the parent class of SimpleDateFormat. If you are using SimpleDateFormat, you should not need a DateFormat object as well (at least not for your purposes).
You are receiving the error most likely because in your SimpleDateFormat, you specified a 3 digit month (MMM), while you are providing a two digit month in the date text you are providing (08). From the API:
Month: If the number of pattern letters is 3 or more, the month is interpreted as text; otherwise, it is interpreted as a number.
This means that MMM is allowed, but it is expecting the name of a month, not the digits for the month. So, it is expecting "2013 Sep 15" instead of "2013 08 15" (I'm not sure if 08 is September or not, it depends on whether or not the DateFormat treats months like indexes or just numbers).