PhpDig checks the presence of tables in config.php. It's the very last statement in that file. I don't see that sites_days_upd is being referenced in 1.8.5 in the config file, so I'm not sure why you're getting that error.
If you run both update_db_to_1_8_4.sql and update_db_to_1_8_5.sql to update your database, you shouldn't have a problem like this.
BTW, I'm in the process of upgrading right now to 1.8.5, so this is the blind leading the blind!
