calDateTime::GetJsDate/SetJsDate are the only major uses of JSAPI in comm-central, and thus susceptible to rooting issues. However, the functions are pretty simple, and the only functions which could GC are JS_NewDateObject*, js::CheckedUnwrap, and JS_AutoCompartment. Of those, the only use we need to keep alive is JSObject *dobj, which crosses the AutoCompartment.
As far as I understand it, this was fixed by bug 918819.
Great! I'm glad our handlification efforts both uncovered and solved this without further analysis.