get OpenSSL certificate lifetime in days.

cert-lifetime.rb

#!/usr/bin/ruby

# took many ideas from https://spin.atomicobject.com/2016/02/03/ssl-certificate-expiration/

require 'openssl'
require 'socket'

class OpenSSLCert

	def initialize hostname
		@hostname=hostname
		@cert=self.get_cert hostname
	end
	
	def get_cert hostname
		tcpsocket = TCPSocket.new(hostname, 443)
		sslsocket = OpenSSL::SSL::SSLSocket.new(tcpsocket)
		sslsocket.hostname=hostname
		sslsocket.connect
		cert = sslsocket.peer_cert
		sslsocket.sysclose
		tcpsocket.close
		return cert
	end
	
	def cert
		return @cert
	end
	
	def expire_date
		@cert.not_after
	end
	
	# how many days before cert expire
	def lifetime
		((self.expire_date.to_i - Time.now.to_i) /(3600*24)).to_i
	end
end

puts OpenSSLCert.new('gitlab.com').lifetime