From Ivica Puljic.

Original javascript from date_popup module was incompatible with
hierarchical_select javascript code. Most likely effects.core.js from
hierarchical_select is in conflict with relativly old ui.calendar.js js control.
I fixed the problem whit hacking date_popup module:
- First I replaced ui.calendar.js control with new ui.datepicker.js control
- Then I hacked date_popup.module - minimal hacking to alow new js code to work.

IMPORTANT: date_popup.module is just minimaly hacked - so lots of config options
for this module for calendar control is not working - for example date format...