Im trying to create a p12 certificate from developer account with a higher expiration date. Currently all my p12 are only valid for 1 year. I believe it can be changed from Certificate Assistant. But not sure of the steps. Any insights would much appreciate.
You cannot change the validity period of an existing Apple Developer certificate or specify a custom validity period when requesting a new certificate.
The validity period of your certificate is governed by the Apple Worldwide Developer Relations Certification Authority. More info in the Certification Practice Statement.
As specified in RFC2986, a Certificate Signing Request (CSR) does not provision a field for the requester to define a validity period. So technically there is no way to tell the Certificate Authority (CA) that you want a custom validity period.
To avoid confusion I will mention that OpenSSL does have a -days option to specify the certificate validity, but only in combination with the -x509 option which issues a self-signed certificate for testing purposes.