SAML assertions are always in UTC. Which is what PingFed is using. Set your Rails application to use UTC as well.
I don't see why you think that the time is off. Your time from Rails is 2015 and -7. That means in UTC, it's tomorrow at 0315 - when it was issued. PingFed is setting an allowance of +/-5 minutes, so the SP should not accept it before 0310 or after 0320.
Your server and PingFed are correct so far...