Date.yesterday is a lie
Rails provides a handy 'Date.yesterday' method. This effectively uses 'Time.zone.today' to calculate yesterday as per the configured timezone.
Ruby's 'Date.today' method uses the system's timezone via a call to localtime(). Therefore, if your Rails app is configured in UTC and you live on the West Coast, tests that assert that 'yesterday < today' will start failing at around 5pm.