At least when using the HTML4 version of the UTCDateBoxImpl -- UTCDateBoxImplHtml4 -- events are not fired for null date/time values. This case can arise when the value is empty or otherwise not a valid date value (as entered by the user).
It is possible to access the HTML4 DateBox field implementation and ask for null value events via DateBox's setFireNullValues, passing true; but this causes an error later, because the adapter event listener code in UTCDateBoxImplHtml4 does not gracefully handle the null value returned in that case by UTCDateBox.date2utc.
I don't know for sure whether the handling in UTCDateBoxImplHtml5 is better, but a quick inspection reveals that it may be: it doesn't appear to require any special handling to trigger ValueChangeEvents for null values, and if a null value were provided, it looks like it would be handled correctly.
At least when using the HTML4 version of the UTCDateBoxImpl -- UTCDateBoxImplHtml4 -- events are not fired for null date/time values. This case can arise when the value is empty or otherwise not a valid date value (as entered by the user).
It is possible to access the HTML4 DateBox field implementation and ask for null value events via DateBox's setFireNullValues, passing true; but this causes an error later, because the adapter event listener code in UTCDateBoxImplHtml4 does not gracefully handle the null value returned in that case by UTCDateBox.date2utc.
I don't know for sure whether the handling in UTCDateBoxImplHtml5 is better, but a quick inspection reveals that it may be: it doesn't appear to require any special handling to trigger ValueChangeEvents for null values, and if a null value were provided, it looks like it would be handled correctly.