EMET detected certificate changed on mail.yahoo.com to a very short-lived certificate from a new CA, issue?

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I just installed EMET with certificate pinning and recorded all of the certs for mail.yahoo.com. Today EMET is notifying me that a certificate issued by VeriSign (which was not a CA that you had certs from last week) has a certificate for *.mail.yaho.com with a very short timeframe Valid from 2/24/2014 to 3/14/2014. Is this legitimate?

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Honesty I havr no idea what is this warning of EMET is about. I never had any warning of this kind related to any serious website, including Yahoo.. Might be some guide to know what platform and browser are you using. Also, my experience is that this kind of "warnings" appear when the computer clock is set on an incorrect date, time or timezone.
So the first thing I would do is check time in Control Panel. If you appreciate any serious difference with local legal time or timezone, tha't the issue.
Else, try to access Yahoo with another browser. If you are using Windows, you should have a functional Internet Explorer installed even if your active browser is another one. So if you are using Firefox, try IE; if you are using Google Chrome, download and install Firefox etc.
In any case, I can assure you that currently there is no problem with Yahoo's certificate