* 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.cc

@@ -15,6 +15,7 @@ GCOptions::GCOptions()
     maxFreed = ULLONG_MAX;
     maxLinks = 0;
     useAtime = false;
+    maxAtime = (time_t) -1;
 }