When authenticating against Autodesk servers using your key & secret, you may get the following message on the response content:
xoauth_problem=system_error&xoauth_additional_error_info= timestamp_mismatch&oauth_error_message=This%20 message%20has%20a%20timestamp%20of%201%2F30 %2F2015%204%3A07%3A30%20PM%2C%20which%20is %20beyond%20the%20allowable%20clock%20skew%20for %20in%20the%20future.
Which can be URL decoded into:
xoauth_problem=system_error&xoauth_additional_error_info= timestamp_mismatch&oauth_error_message=This message has a timestamp of 1/30/2015 4:07:30 PM, which is beyond the allowable clock skew for in the future.
Which basically means the clock of your computer (or server) where the code is running is not correct. To fix it, simply adjust the clock.