If the parameter is specified, the module outputs the data for the subsequent era; if the parameter is specified it outputs the data for the previous one. If the parameter is specified, the module outputs the data for the "old" era. This is the same as the current era unless the year is set to the first year of the specified era. If this is the case, then outputs the data for the previous era. However, if the module could not find a valid previous era then the data for the current era is returned.