In Yahoo! Mail Classic you have the advanced search options (just click search without entering any criteria in the search box to get the advanced options page). From there you can search for all emails after a certain date (ie. a date in the future). In the search results you can select them all and mark as spam / move to trash etc.
However, I don't see how you can get to the same advanced search options in the New Mail interface? You must initially search for something in order to get the "Narrow Results" options on the right of the page (for me there is no Advanced Search link at the bottom as is mentioned in the help pages). You can then pick the year (2038, if there are emails in the search results from that year) then select all the results and flag as spam / move to trash etc. as you would for Mail Classic.