Hi Nina,
There are 3 places Hub looks when writing the "Last Updated" date. If an admin has explicitly set a revised 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).
For items in your site, there appear to be two issues, one of which is a bug we need to fix in Hub. First, we have observed a time zone error causing update dates to be off by a day. We are working to fix this bug, but in the meantime you may see update dates like this one, where the the revision date is "2019-01-02", but the update date is displayed as "1/1/2019": https://data-wi-dnr.opendata.arcgis.com/datasets/wisconsin-dem-and-hillshade-from-lidar-index.
Second, from the edge case example you mentioned, the revision date of "2002" is not being read as a valid date, which is why the item is displaying "12/31/1969" as the update date. If you change this to something like "2019-03-20" it should fix the problem. I would recommend using the calendar date picker as it's a bit easier to ensure formatting is correct:
Apologies for these issues, but I hope this information helps. We are working hard to resolve the bug and will let you know when we're done.
Best,
Patrick