Merge pull request #3957 from mitchellh/clockskewhelp

add some debugging info for if your clock is skewed

website/source/docs/builders/amazon.html.md

@@ -155,3 +155,17 @@ work, but specifics will depend on your use-case.
     ]
 }
 ```
+
+### Checking that system time is current
+
+Amazon uses the current time as part of the [request signing
+process](http://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html). If
+your system clock is too skewed from the current time, your requests might
+fail. If that's the case, you might see an error like this:
+
+    ==> amazon-ebs: Error querying AMI: AuthFailure: AWS was not able to validate the provided access credentials
+
+If you suspect your system's date is wrong, you can compare it against
+http://www.time.gov/. On Linux/OS X, you can run the `date` command to get the
+current time. If you're on Linux, you can try setting the time with ntp by
+running `sudo ntpd -q`.