Thanks for replying on this thread, Mark. Hopefully I can provide some clarification about how dates work in Hub.
There are 3 places Hub looks when writing the "Last Updated" date. If an admin has explicitly set a publication date in the metadata editor in ArcGIS Online, we will use that date. If not, then depending on whichever was updated more recently we will use either a) the last time the item was edited or “touched” in ArcGIS Online or b) the last time an update was made to the service (if editor tracking is enabled).
You can test this on sites with the new search enabled (e.g. https://hub.arcgis.com/search) by hovering over the date field in search. If the tooltip reads “Last updated” then it’s coming from the publication date in the metadata editor. If it reads “Metadata updated” then the date is coming from the last time the item was edited in ArcGIS Online. If it reads “Data updated” then the date is coming directly from the service.