The date filters seem to be ignored for certain search terms and not others. Searching for, say, "dune before:2019-04-05" filters as expected, but searching for current events proximate terms like "iran" or "donald" returns results that disregard the date ceiling completely.
I confirmed searching for "Iran" with a before set ends up showing me all recent results. BUT if i use this search form to generate the query (and not the youtube search bar) it works.
via the search form: https://www.youtube.com/results?search_query=+%22iran%22+bef...
via the YT search: https://www.youtube.com/results?search_query=+%22iran%22+bef...