certification used by apache2 can be generated by "make-ssl-cert". however, the period is only for 1 month. it's too short for most administrators. therefore, we can modify make-ssl-cert to expend the validation period.
$ sudo vim /usr/sbin/make-ssl-cert
find "-keyout $output", append "-days 365" in the tail. than execute make-ssl-cert, the period will by one year.