* GC option `--max-atime' that specifies an upper limit to the last

accessed time of paths that may be deleted.  Anything more recently
  used won't be deleted.  The time is specified in time_t,
  e.g. seconds since 1970-01-01 00:00:00 UTC; use `date +%s' to
  convert to time_t from the command line. 

  Example: to delete everything that hasn't been used in the last two
  months:

  $ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")

src/libstore/store-api.hh

@@ -75,6 +75,10 @@ struct GCOptions
        atime. */
     bool useAtime;
 
+    /* Do not delete paths newer than `maxAtime'.  -1 means no age
+       limit. */
+    time_t maxAtime;
+
     GCOptions();
 };