I'm reporting an issue in 3.7.2 that I've patched locally. I patched the minimized version> Unfortunately, I don't have the time to hook up the source version and test to find the definitve method to report, but I'm posting my finding here anyways as a (quasi) public service in case the issue exists with later versions that may need the patch…

When sorting on a custom date I get an error on line 19 in jQuery.jqGrid.min.js:

function(J){J||(J="");return b.trim(J.toUpperCase()};

I get an object does not support property or method error in IE. The value of J when the error occurs is “Thu Jun 16 00:00:00 EDT 2011” but it's (apparently) not a String. The simple fix is:

function(J){J||(J="");return b.trim(("" + J).toUpperCase())};

Making sure that J evaluates to a string by casting it: