diff --git a/owlps-positioner/src/timestamp.cc b/owlps-positioner/src/timestamp.cc
index 70dcee6..587eef5 100644
--- a/owlps-positioner/src/timestamp.cc
+++ b/owlps-positioner/src/timestamp.cc
@@ -6,6 +6,7 @@
 
 
 #include "timestamp.hh"
+#include "configuration.hh"
 
 #include <boost/functional/hash.hpp>
 
@@ -92,6 +93,8 @@ inline void Timestamp::set(const uint64_t source_ms)
 
 Timestamp Timestamp::get_current_time()
 {
+  if (! Configuration::is_configured("replay"))
+    current_time.now() ;
   return current_time ;
 }
 
@@ -100,12 +103,14 @@ Timestamp Timestamp::get_current_time()
  * @source A pointer to the new time. If NULL, #current_time is updated
  * to the current time.
  */
-void Timestamp::set_current_time(const Timestamp *const source)
+void Timestamp::update_current_time(const Timestamp &source)
 {
-  if (source)
-    current_time = *source ;
-  else
-    current_time.now() ;
+  // We should never call this function if we are in replay mode:
+  assert(Configuration::is_configured("replay")) ;
+
+  // According to human perception, time generally goes only forward:
+  if (current_time < source)
+    current_time = source ;
 }
 
 
diff --git a/owlps-positioner/src/timestamp.hh b/owlps-positioner/src/timestamp.hh
index 2b36244..a15167b 100644
--- a/owlps-positioner/src/timestamp.hh
+++ b/owlps-positioner/src/timestamp.hh
@@ -69,7 +69,7 @@ public:
   /// Returns a Timestamp containing the current time
   static Timestamp get_current_time(void) ;
   /// Sets #current_time to a new timestamp
-  static void set_current_time(const Timestamp *const source = NULL) ;
+  static void update_current_time(const Timestamp &source) ;
   /// \brief Initialises #timestamp at the current time whith a
   /// nanosecond precision
   bool now(void) ;