cert_expire.rb

require 'date'
Facter.add(:cert_expire) do
  confine :kernel => 'Linux'
  setcode do
    hostname = Facter.value(:hostname)
    info = Facter::Core::Execution.execute("echo | openssl s_client -showcerts -connect #{hostname}:443 2>/dev/null | openssl x509 -noout -dates")

    expire_info = {}

    dates = info.split("\n")

    if !dates[0].nil?
      start_date = dates[0].gsub('notBefore=','')
      end_date = dates[1].gsub('notAfter=','')

      expire_info['start_date'] = start_date
      expire_info['end_date'] = end_date
      expire_info['days_to_expire'] = (Date.parse(end_date) - Date.today).to_i
    else
      expire_info = ''
    end

    expire_info
  end
end

Here ya go @sshastri

require 'date'
Facter.add(:cert_expire) do
  confine kernel: :Linux

  setcode do
    hostname = Facter.value(:hostname)

    cert_info = Facter::Core::Execution.execute("echo | openssl s_client -showcerts -connect #{hostname}:443 2>/dev/null | openssl x509 -noout -dates")

    expire_info = {}
    dates = cert_info.split("\n")

    unless dates[0].nil?
      start_date = dates[0].gsub('notBefore=', '')
      end_date = dates[1].gsub('notAfter=', '')

      expire_info['start_date'] = start_date
      expire_info['end_date'] = end_date
      expire_info['days_to_expire'] = (Date.parse(end_date) - Date.today).to_i
    end

    expire_info
  end
end