def expires_at(options, default = @default_expires)
      value = expires_value(options, default)
      Numeric === value ? Time.now + value : value
    end
def expires_at(options, default = @default_expires)
      value = expires_value(options, default)
      Numeric === value ? Time.now + value : value
    end
Calculates the time when something will expire.

 This method considers false and 0 as "no-expire" and every positive
 number as a time to live in seconds.

 @param [Hash] options Options hash
 @option options [0,false,nil,Numeric] :expires expires value given by user
 @param [0,false,nil,Numeric] default default expiration time

 @return [false] if it should not expire
 @return [Time] the time when something should expire
 @return [nil] if it is not known