Im not sure about ExtJS specifically but there's a very similar question asked here and the answer links to the documentation; it's worth checking that question out just for the example in the answer though!
In a pure Javascript implementation a regex may be the way to go. There's quite a few Regex Examples available and even a few Generators which can make your life easier if you aren't too confident with regex. Here's an example of the generated code from that site - jsFiddle link. (Thats not the cleanest way of doing a regex, when you use a generator it actually leaves quite a lot to be desired unfortunately. But it does work and give a decent starting point!)
Upon closer inspection of the string, I notice it uses text to signify the month. This could be worked around via the use of an enum.
As far as ExtJS goes - there's a pretty good blog post here about some of the date format options. But I think the documentation linked at the top of the answer is the best place to be looking.