Handle clock mismatch for API signature

[AaronAtDuo]

Detailed Description

Now that Duo Unix is including the datestamp in the API signature, API calls will fail if the server and Duo disagree too much on the current time. It would be useful to use the /ping endpoint to determine Duo's server time, and offset the datestamp accordingly, so that the API will still work for customers whose server time is a little off.

Use Case

Makes the API more accessible to customers.

Workarounds

Otherwise customers must keep their server time more in-line with Duo.