Strange - when you change the date and then save the document the index is updated automatically - did you change via agent or something?

There is a manual index update as Leonardo said - via Quick Menu - "Rebuild Site Indexes".

Were changes made to a local replica and replicated to a server or made direct to the server version? - When replicating from local the indexes on the server version will not update immediately - but it will only be about 5 minutes before they are (so that is not the issue for you).