In LZX reference the example for datepicker contains a wrong date for the selecteddate attribute. I guess selecteddate should be after inbetween earliestdate and latestdate.

Location: openlaszlo-4.0.0/docs/reference/datepicker.html

Code:
<datepicker showingdate="new Date(2006,3,1)"
              earliestdate="new Date( 2003, 9, 5)"
              latestdate="new Date( 2005, 1, 5)"
              selecteddate="new Date( 2006, 3, 20)">

Should be something like:
              <datepicker showingdate="new Date()"
              earliestdate="new Date( 2007, 1, 1)"
              latestdate="new Date( 2008, 11, 31)"
              selecteddate="new Date()">