-stl (Set archive timestamp from the most recently modified file) switch
Syntax
-stl
 If -stl switch is specified, 7-Zip sets timestamp for archive file as 
timestamp from the most recently modified file in that archive.
Examples
7z u a.7z -stl *.txt
Commands that can be used with this switch
  a (Add),
  d (Delete),
  rn (Rename),
  u (Update)