Expired certificates are slightly trickier. Which certificate generator are you using (Tools > Fiddler Options > HTTPS, look at the blue link on the right)?
Probably the simplest
thing to do would be to use the app so that Fiddler creates the certificate, then, without closing the app, change the local system clock to three years in the future.
Alternatively, if you're using the CertEnroll generator, you can type
prefs set fiddler.certmaker.ValidDays -364
in the QuickExec box so that the certificates generated expire nearly a year before the current date.
If you do that, don't forget to remove the preference and use the "Remove Interception Certificates" Action in the HTTPS tab to remove the "bad" certificates after you're done testing (or everything else will break too!)
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