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NET:ERR_CERT_DATE_INVALID

If you are unable to do so, you can also update the security certificate directly. Here are instructions on how to do so:

1. Download Let'sEncrypt ISRGRootX1 certificate from letsencrypt.org - https://letsencrypt.org/certs/isrgrootx1.der. You should only get the certificate from letsencrypt.org. You'll have to do this on a device other than the one running Yosemite / El Capitan, since the updated certificate is on a site that is inaccessible to browsers that rely on the old certificate.
2. Copy isrgrootx1.der to the Mac running the old version of Mac OS.
3. Open the Keychain Access application. It is in the Utilities folder inside the Applications folder.
4. From the File menu choose Import Items.
5. Navigate to the isrgrootx1.der file and select it. In the Destination Keychain pop-up menu, choose the System keychain, then click Open. Enter your administrator password if prompted.
6. Select the System keychain in the Keychains section of the Keychain Access window.
7. Find the ISRG Root X1 cert in the System keychain list and double-click on it.
8. Click the disclosure triangle to expand the Trust section. Under When using this certificate select Always Trust. Close the window. You may need to enter your administrator password.
9. Safari should immediately start working, although you may need to reload open pages. For other browsers, we recommend you restart your machine before attempting to access Reverb.