diff --git a/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c b/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
index ddb8271..22d7de1 100644
--- a/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
+++ b/infrastructure-centred/owlps-aggregator/owlps-aggregatord.c
@@ -413,7 +413,7 @@ void* monitor_couples()
   TIMESTAMP current_time ;
   FILE *fd = NULL ;
   char *mac_string ;
-  uint_fast32_t sub ; // time_elapsed() result
+  uint_fast32_t sub ; // owl_time_elapsed_ms() result
 
   uint_fast32_t aggregate_timeout =
     cfg_getint(cfg, "aggregate_timeout") ;
@@ -453,7 +453,8 @@ void* monitor_couples()
 
       while (couple_ptr != NULL) // Parsing list
         {
-          sub = owl_time_elapsed(couple_ptr->start_time, current_time) ;
+          sub = owl_time_elapsed_ms(couple_ptr->start_time,
+                                    current_time) ;
 
           // If the couple was not treated already
           if (couple_ptr->info != NULL)
@@ -632,8 +633,8 @@ void got_couple_info(couple_message message)
             { // Research criterion: MAC and transmission time
               if (owl_mac_equals(message.mobile_mac_addr_bytes,
                                  tmp_couple->mobile_mac_addr_bytes) == 1
-                  && owl_time_elapsed(message.request_time,
-                                      tmp_couple->request_time) == 0)
+                  && owl_time_elapsed_ms(message.request_time,
+                                         tmp_couple->request_time) == 0)
                 break ; // If the couple exists, we stop on it
               tmp_couple = tmp_couple->next ;
             }
@@ -647,8 +648,8 @@ void got_couple_info(couple_message message)
               // TODO : define an option for the maximal difference time.
               if (owl_mac_equals(message.mobile_mac_addr_bytes,
                                  tmp_couple->mobile_mac_addr_bytes) == 1
-                  && owl_time_elapsed(message.start_time,
-                                      tmp_couple->request_time) <= 10)
+                  && owl_time_elapsed_ms(message.start_time,
+                                         tmp_couple->request_time) <= 10)
                 break ; // If the couple exists, we stop on it
               tmp_couple = tmp_couple->next ;
             }
@@ -919,7 +920,7 @@ void delete_old_aps()
   owl_timestamp_now(&now) ;
 
   while (token_aps != NULL)
-    if (owl_time_elapsed(token_aps->last_seen, now) >
+    if (owl_time_elapsed_ms(token_aps->last_seen, now) >
         (uint_fast32_t) cfg_getint(cfg, "ap_keep_timeout") * 1000)
       delete_ap(token_aps) ;
     else
diff --git a/libowlps/libowlps.c b/libowlps/libowlps.c
index 4b4151e..56dabf0 100644
--- a/libowlps/libowlps.c
+++ b/libowlps/libowlps.c
@@ -163,14 +163,26 @@ void owl_timestamp_to_string(char *dst, TIMESTAMP src)
 /*
  * Returns the time (in milliseconds) between two dates.
  */
-uint_fast32_t owl_time_elapsed(TIMESTAMP sup, TIMESTAMP inf)
+uint_fast32_t owl_time_elapsed_ms(const TIMESTAMP d1, const TIMESTAMP d2)
 {
-  uint_fast32_t elapsed_ms =
-    abs(owl_timestamp_to_ms(sup) - owl_timestamp_to_ms(inf)) ;
+  return owl_timestamp_to_ms(owl_time_elapsed(d1, d2)) ;
+}
+
+
+
+/*
+ * Returns a TIMESTAMP storing the time between two dates.
+ */
+TIMESTAMP owl_time_elapsed(const TIMESTAMP d1, const TIMESTAMP d2)
+{
+  TIMESTAMP elapsed ;
+  elapsed.tv_sec = abs(d1.tv_sec - d2.tv_sec) ;
+  elapsed.tv_nsec = abs(d1.tv_nsec - d2.tv_nsec) ;
 #ifdef DEBUG
-  printf("time_elapsed(): %"PRIuFAST32"ms\n", elapsed_ms) ;
+  printf("time_elapsed(): %"PRIu64"ms\n",
+         owl_timestamp_to_ms(elapsed)) ;
 #endif
-  return elapsed_ms ;
+  return elapsed ;
 }
 
 
diff --git a/libowlps/owlps.h b/libowlps/owlps.h
index ca66e4e..0c01cda 100644
--- a/libowlps/owlps.h
+++ b/libowlps/owlps.h
@@ -234,7 +234,8 @@ TIMESTAMP owl_timespec_to_timestamp(const struct timespec d) ;
 TIMESTAMP owl_timeval_to_timestamp(const struct timeval d) ;
 void owl_timestamp_to_string(char *dst, TIMESTAMP src) ;
 uint64_t owl_timestamp_to_ms(TIMESTAMP date) ;
-uint_fast32_t owl_time_elapsed(TIMESTAMP sup, TIMESTAMP inf) ;
+uint_fast32_t owl_time_elapsed_ms(const TIMESTAMP d1, const TIMESTAMP d2) ;
+TIMESTAMP owl_time_elapsed(const TIMESTAMP d1, const TIMESTAMP d2) ;
 TIMESTAMP owl_hton_timestamp(TIMESTAMP date) ;
 TIMESTAMP owl_ntoh_timestamp(TIMESTAMP date) ;