'Date modified' could show only the time when it was modified today, show date without year (day/month or month/day depending on localization) when it's modified this year and complete date when it's modified on an other year (day/month/year or month/day/year depending on localization).
Committed with revision 22534. 2006-07-27 Benedikt Meurer <benny@xfce.org> * thunar/thunar-file.c, thunar/thunar-util.{c,h}: Use localized human readable format for file dates. Bug #2057.