validate() relied on this.value, which is only set by _inputChanged on
the change event. Submitting via Enter while focus is still in an input
field skips that event, leaving this.value undefined and causing valid
dates to fail validation.
Introduce _valueFromInputs() as the single parsing source: it reads the
three sub-inputs directly and returns a UTC-midnight DateTime.
validate()
now falls back to it when this.value is not set. _inputChanged is
refactored to call _valueFromInputs() as well, removing the duplicated
format-mapping logic.
pk