Time stamped indices use your selected time range to create a list of indices that match a specified timestamp pattern. This can be very efficient for some data sets (eg, logs) For example, to match the default logstash index pattern you might use [logstash-]YYYY.MM.DD. The [] in "[logstash-]" are important as they instruct Kibana not to treat those letters as a pattern. Please also note that indices should rollover at midnight UTC.

See http://momentjs.com/docs/#/displaying/format/ for documentation on date formatting.