This FiddlerScript highlights in red Sessions secured by certificates that will expire in the next 30 days

WarningBeforeCertExpires.js

// Inside Rules > Customize Rules > OnBoot, add the following line:
	FiddlerApplication.add_OnValidateServerCertificate(onEvalCert);
	
// Just before that function, add the following new function:
	static function onEvalCert(o: Object, e: ValidateServerCertificateEventArgs)
	{
		try
		{
			var X2: System.Security.Cryptography.X509Certificates.X509Certificate2 = 
				new System.Security.Cryptography.X509Certificates.X509Certificate2(e.ServerCertificate);
			
			// Cert expires in next 30 days
			if (DateTime.UtcNow.AddDays(30) > X2.NotAfter)
			{
				if (DateTime.UtcNow.AddDays(5) > X2.NotAfter)
				{
					FiddlerApplication.AlertUser("Certificate Expiring!",
					"The certificate for " + e.Session.hostname + " is expiring " + X2.NotAfter.ToShortDateString() + "!");			
				}
				e.Session["ui-backcolor"] = "red";
					
				e.Session["ui-Comments"] = "Cert Expires:" + e.ServerCertificate.GetExpirationDateString();
				e.Session.RefreshUI();
			}
		}
		catch (ex)
		{
			FiddlerApplication.Log.LogFormat("Failed to evaluate certificate: {0}", ex.Message);
		}
	}