are you positive that there are results in the database for the USERIDENT you are testing where the comments.date = uploads.date?
I find it helpfull when troubel shooting SQL Queries to take the web page out of the mix and go do the query right in the database. If you have a local database, use a tool like the MySQL GUI Tools query browser or SQLYog to access the database and run the query.
first try with just the date aspect:
SELECT comments.*, uploads.date
FROM comments, uploads
WHERE comments.date=uploads.date
see if this returns a result, if so make note of one of the USERIDENT values and try the query with one of those values:
SELECT comments.*, uploads.date
FROM comments, uploads
WHERE USERIDENT = <value from precious query> AND comments.date=uploads.date
if the first query does not return a result, double check the values being stored in the comments.date and uploads.date columns.
