When I have on page date picker P4_MY_DATE and want to get its value with JS call apex.items.P4_MY_DATE.value result is undefined. When I set P4_MY_DATE on page load, apex.items.P4_MY_DATE.value will return value but it is always what was set on page load.
When using function apex.items.P4_MY_DATE.getValue() all is OK, so there is workaround.