In this case, I would still recommend you to install Microsoft CA instead of seflssl which design for only local machine SSL usage. You can't trust it't CA root as there's none, and the cert is generated and inserted directly to website rather than a file. What you can do of couse is to use cert mmc then export out the cert, which is in pfx format. Not x509. You can specify the expiry date to a long time using /V switch. Again that's not the point, you would be better off with Microsoft CA, which allow you to issue cert, download CA root cert in x.509 format, as well as the cert expiry date.
Cheers,
Bernard Cheah