require 'time'

rfc2822_time = 'Sun, 31 Aug 2008 12:08:19 +0900'
t = Time.rfc2822(rfc2822_time)
p t.kind_of?(Time) #=> true
non_rfc2822_time = '31 Aug 2008 12:08:19 +0900'

begin
  p Time.rfc2822(non_rfc2822_time)
rescue ArgumentError => err
  puts "format err: #{err}"
end