chunks+peek+ping: use correct underlying type for time::milliseconds

Change-Id: I3f46f846401eccc0c11d3abcd1a5ebff89056f7d
diff --git a/tools/chunks/putchunks/main.cpp b/tools/chunks/putchunks/main.cpp
index c92182e..5e1dd9f 100644
--- a/tools/chunks/putchunks/main.cpp
+++ b/tools/chunks/putchunks/main.cpp
@@ -109,7 +109,7 @@
   }
 
   opts.freshnessPeriod = time::milliseconds(vm["freshness"].as<time::milliseconds::rep>());
-  if (opts.freshnessPeriod < time::milliseconds::zero()) {
+  if (opts.freshnessPeriod < 0_ms) {
     std::cerr << "ERROR: FreshnessPeriod cannot be negative" << std::endl;
     return 2;
   }