Print how many days left until an SSL certificate file expires. Usage: ssl_expire_days CERTFILE

ssl_expire_days.sh

#!/bin/sh                                                                                                                         

ssl_expire_days() {
    # Show usage info if not called with a certificate file                                                                       
    if [ $# -eq 0 ]; then
        echo "Usage: ssl_expire_days CERTFILE"
        return 0
    fi

    cert=$1
    cert_expire_date=$(openssl x509 -noout -in ${cert} -enddate | sed 's/^[a-zA-Z]*=//g')
    expires_in_days=$(( ($(date -d "${cert_expire_date}" +%s) - $(date +%s)) / 86400))

  echo ${expires_in_days}
}

ssl_expire_days $1

Take a look at https://gist.github.com/simonwhitaker/4179149 if your looking to check the expire date of a domain instead of a file