Using the Behaviours plugin to determine is a date field is NULL and if it is we need to copy the value of an existing date field into it. The below only return a value of object instead of the date value
def OriginalDate = getFieldById("customfield_16324")
def ExpectedDate = getFieldById("customfield_16325")
def ODate = OriginalDate.getValue()
def EDate = ExpectedDate.getValue()
if ( ! EDate ) {
ExpectedDate.setFormValue{ODate}
}