Edit the tiddler called SideBarOptions. (You can find it under the Shadowed tab under the More tab). Change the date format in the NewJournal macro. For example:

<<newJournal 'YYYY-MM-DD'>>

See also DateFormats
bag
tiddlywiki_public
created
Sun, 20 Mar 2011 05:29:15 GMT
creator
vivekkodira
modified
Sun, 20 Mar 2011 05:29:15 GMT
modifier
vivekkodira