* 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/gc.cc

@@ -531,10 +531,11 @@ void LocalStore::gcPathRecursive(const GCOptions & options,
     startNest(nest, lvlDebug, format("looking at `%1%'") % path);
         
     /* Delete all the referrers first.  They must be garbage too,
-       since if they were in the closure of some live path, then this
+       since if they were live, then the current path would also be
-       path would also be in the closure.  Note that
+       live.  Note that deleteFromStore() below still makes sure that
-       deleteFromStore() below still makes sure that the referrer set
+       the referrer set has become empty, just in case.  (However that
-       has become empty, just in case. */
+       doesn't guard against deleting top-level paths that are only
+       reachable from GC roots.) */
     PathSet referrers;
     if (isValidPath(path))
         queryReferrers(path, referrers);
@@ -571,7 +572,7 @@ struct CachingAtimeComparator : public std::binary_function<Path, Path, bool>
 string showTime(const string & format, time_t t)
 {
     char s[128];
-    strftime(s, sizeof s, format.c_str(), gmtime(&t));
+    strftime(s, sizeof s, format.c_str(), localtime(&t));
     return string(s);
 }
 
@@ -766,7 +767,12 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
             while (!prioQueue.empty()) {
                 checkInterrupt();
                 Path path = prioQueue.top(); prioQueue.pop();
-                printMsg(lvlInfo, format("deleting `%1%' (last accessed %2%)") % path % showTime("%F %H:%M:%S", atimeComp.cache[path]));
+
+                if (options.maxAtime != (time_t) -1 &&
+                    atimeComp.lookup(path) > options.maxAtime)
+                    continue;
+                
+                printMsg(lvlInfo, format("deleting `%1%' (last accessed %2%)") % path % showTime("%F %H:%M:%S", atimeComp.lookup(path)));
 
                 PathSet references;
                 if (isValidPath(path)) references = queryReferencesNoSelf(path);

src/libstore/store-api.cc

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

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();
 };
 

src/nix-store/nix-store.cc

@@ -531,8 +531,14 @@ static void opGC(Strings opFlags, Strings opArgs)
         else if (*i == "--max-freed") options.maxFreed = getIntArg(*i, i, opFlags.end());
         else if (*i == "--max-links") options.maxLinks = getIntArg(*i, i, opFlags.end());
         else if (*i == "--use-atime") options.useAtime = true;
+        else if (*i == "--max-atime") {
+            options.useAtime = true;
+            options.maxAtime = getIntArg(*i, i, opFlags.end());
+        }
         else throw UsageError(format("bad sub-operation `%1%' in GC") % *i);
 
+    if (!opArgs.empty()) throw UsageError("no arguments expected");
+    
     PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
     store->collectGarbage(options, results);
 
@@ -745,7 +751,7 @@ void run(Strings args)
             indirectRoot = true;
         else if (arg[0] == '-') {            
             opFlags.push_back(arg);
-            if (arg == "--max-freed" || arg == "--max-links") { /* !!! hack */
+            if (arg == "--max-freed" || arg == "--max-links" || arg == "--max-atime") { /* !!! hack */
                 if (i != args.end()) opFlags.push_back(*i++);
             }
         }