Command notafter sends notifications via mail(1) if TLS certs for the specified domains will expire soon or have expired. For more details, see the package documentation.

The command is called "notafter" because it works on the "notAfter" date on certificates.

With Go 1.19 or higher:

go install github.com/nishanths/notafter@latest

See notafter -h. You may want to run the command regularly as a part of a cron or a recursive at(1) job.