The problem with your query is that, you are using a php date object inside the query and hence the query will always fail and that is the reason why you get the warning message.
You can simply use the NOW() function of mysql to accomplish this, instead of constructing a date object in php and passing it to mysql. So your query will look like:
$column_name - NOW() > 0
Also FYI, to debug such errors, it is always a good practice to include an error message printing when a query files. You can do that using the following code whenever you do a mysql call:
Using this will tell you exactly where the error in your query is instead of just printing a warning message.