It appears that jqModal and datePicker have z-index problems. My datepicker appeared below the form inside jqModal. The fix for this is very simple. We have to modify CSS for UI and jqmodal.
This is because they don't set z-index property properly. But we can change it on our own...

Inside your jqModal class (i.e. jqWindow or something equivalent) add:

z-index: 200;

for example:

div.jqmAlert { /* contains + positions the alert window */
  display: none;
  z-index: 200;
  position: fixed;
  top: 7%;
  width: 100%;
}

and then in the jQuery UI CSS change the line:

.ui-datepicker { width: 17em; padding: .2em .2em 0;  }

to

.ui-datepicker { width: 17em; padding: .2em .2em 0; z-index: 10000; }

Now it should work, at least for me it did. If it doesn't, fiddle with z-index properties using FireBug.
Good luck!