improve timezone offsets in date pickers

The datepicker for graph panel is extend from extjs's datepicker object. The extended version automatically offsets the date by local time (since javascript "helpfully" adjusts all date objects to local time) and by the date offset set by the user in the zenoss ui.  

The get/set for the dates work as expected, but there are some events that cause the datepicker to get/set values internally. When this happens, the local time offset is applied twice.  

This commit fixes that...