You should understand that when returning search results, Domino allows three basic options: by relevancy, by oldest or by newest. It all depends on the &SearchOrder= value. A value of 1 (e.g., &SearchOrder=1) is by relevancy, SearchOrder=2 is by oldest and SearchOrder=3 is by newest. The problem is that the oldest and the newest is by age of the document since it was last modified. So how can you sort by some other field -- say, PublishDate?
Add a hidden column with the value PublishDate. Then sort it Ascending or Descending (your choice).
Next: When you submit your search query use &SortOrder=4 as part of the query and it will use that hidden column. The other SortOrder values mentioned still work, too.
Do you have comments on this tip? Let us know.