I have been using this plugin for awhile now, and love it. I have the +- 4min option turned on, but it looks like my web server has drifted _just_ over 4min from my phone's time. My phone's clock is synced with the national time service, so can't really do anything there. I manually edited the plugin to allow for a larger time drift (+- 10min), and I was able to log in.
However, if/when the plugin updates, I will need to make these changes again. Ideally, I'd like to see a settings screen that gives the admin control over how much drift is allowable.
In addition, I'd like it if the plugin showed the server time on the login screen (even if it's in a HTML comment). This would help troubleshoot login problems related to GAuth.