Fix return certificate expiry time from NearExpiration (#29128)

* Fix return certificate expiry time from NearExpiration

 - The duration returned from the NearExpiration is supposed to
   represent the time till expiry from now and not the calculated
   time a month from now.

* Add cl

* PR feedback

changelog/29128.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+vault/diagnose: Fix time to expiration reporting within the TLS verification to not be a month off.
+```

vault/diagnose/tls_verification.go

@@ -270,15 +270,17 @@ func TLSFileWarningChecks(leafCerts, interCerts, rootCerts []*x509.Certificate)
 	return warnings, nil
 }
 
-// NearExpiration returns a true if a certficate will expire in a month and false otherwise
+// NearExpiration returns a true if a certificate will expire in a month
+// and false otherwise, along with the duration until the certificate expires
+// which can be a negative duration if the certificate has already expired.
 func NearExpiration(c *x509.Certificate) (bool, time.Duration) {
-	oneMonthFromNow := time.Now().Add(30 * 24 * time.Hour)
-	var timeToExpiry time.Duration
-	if oneMonthFromNow.After(c.NotAfter) {
-		timeToExpiry := oneMonthFromNow.Sub(c.NotAfter)
-		return true, timeToExpiry
-	}
-	return false, timeToExpiry
+	now := time.Now()
+	timeToExpiry := c.NotAfter.Sub(now)
+
+	oneMonthFromNow := now.Add(30 * 24 * time.Hour)
+	isNearExpiration := oneMonthFromNow.After(c.NotAfter)
+
+	return isNearExpiration, timeToExpiry
 }
 
 // TLSMutualExclusionCertCheck returns error if both TLSDisableClientCerts and TLSRequireAndVerifyClientCert are set