If a dataset is continually added to, but the metadata describing it doesn't change, there is no need to record a Date modified.
This information answers the question: When was this metadata record last modified in its source repository? This date does not refer to the date of change in the ANDS Collections Registry itself and is not derived from other data.
It has no relation to the data of the last harvest of metadata from a data source.
Date modified is an indication of the currency of the information in a metadata record.
It records the date metadata describing an activity, collection, party or service was last changed in the source system.