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Investigate http get last modification date field - 60% in HDX have this according to UofV.
References
Using the Update Frequency Metadata Field and Last_update CKAN field to Manage Dataset Freshness on HDX:
https://docs.google.com/document/d/1g8hAwxZoqageggtJAdkTKwQIGHUDSajNfj85JkkTpEU/edit#
University of Vienna paper on methodologies for estimating next change time for a resource based on previous update history:
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