The format chart for the "Date" class indicates that "z" can be used to handle the ordinal day (that is, the day of the year from 0 to 365). It seems like Ext can print the "z" format correctly, but not parse it. For example, this works fine:
Running that today (April 18, 2012) prints "108".
But this does not work:
It simply returns a Date object set to the first day of the year.