Please see https://www.xfinity.com/support/articles/search-your-xfinity-connect-inbox. To search by date, enter a date or range (ex: 3/1/2010 or 1/1/2000-3/12/2010). If you have the newer Open-Xchange "Appsuite" webmail ("connect.xfinity.com/appsuite/..."), the info you entered will be followed by the words "as date" or "as daterange". Click that line. If necessary, select the folder(s) to search.
You may find that the search doesn't find emails in the Trash folder when you search All Folders. If so, you'll need to specify the Trash folder instead of All Folders.
After you run the search and verify the results, check the "Select all" box, and then click the Delete icon.