When creating your own self-signed cert, you can specify how long you want the cert to be valid.... in the following example, 10 years.
I've not used the auto-generation web sites but rather install OpenSSL (windows or linux) and create them myself. The basic steps...
STEP 1:
openssl genrsa -out CertificateAuthority.KEY 4096
STEP 2:
openssl req -new -x509 -days 3650 -key CertificateAuthority.KEY -out CertificateAuthority.CERT -config openssl.cnf
STEP 3:
openssl pkcs12 -export -out CertificateAuthority.p12 -inkey CertificateAuthority.KEY -in CertificateAuthority.CERT
The output files names to contents are:
CertificateAuthority.KEY - text file, private key, no password
-----BEGIN PRIVATE KEY-----
-----END PRIVATE KEY-----
CertificateAuthority.CERT - text file, certificate
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
ClientCertificate.P12 - p12 binary file, certificate
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In depth How-To
How-to: Make Your Own Cert With OpenSSL on Windows
As several things have changed since I published “Howto: Make Your Own Cert With OpenSSL on Windows” 5 years ago, I’m publishing an updated how-to. This time, I’m using the …
blog.didierstevens.com
The above has instructions/links to download, install, configure the OpenSSL software; i.e. openssl.cnf.