diff --git a/local.c b/local.c
index 3619b58..513779e 100644
--- a/local.c
+++ b/local.c
@@ -479,7 +479,7 @@ LCL_NotifyExternalTimeStep(struct timeval *raw, struct timeval *cooked,
     double offset, double dispersion)
 {
   /* Dispatch to all handlers */
-  invoke_parameter_change_handlers(raw, cooked, 0.0, offset, LCL_ChangeStep);
+  invoke_parameter_change_handlers(raw, cooked, 0.0, offset, LCL_ChangeUnknownStep);
 
   lcl_InvokeDispersionNotifyHandlers(dispersion);
 }
diff --git a/local.h b/local.h
index bdf38bd..4000acd 100644
--- a/local.h
+++ b/local.h
@@ -74,7 +74,8 @@ extern void LCL_GetOffsetCorrection(struct timeval *raw, double *correction, dou
 
 typedef enum {
   LCL_ChangeAdjust,
-  LCL_ChangeStep
+  LCL_ChangeStep,
+  LCL_ChangeUnknownStep
 } LCL_ChangeType;
 
 typedef void (*LCL_ParameterChangeHandler)
diff --git a/manual.c b/manual.c
index 011d07a..c286ce8 100644
--- a/manual.c
+++ b/manual.c
@@ -221,6 +221,11 @@ slew_samples(struct timeval *raw,
 {
   double delta_time;
   int i;
+
+  if (change_type == LCL_ChangeUnknownStep) {
+    MNL_Reset();
+  }
+
   for (i=0; i<n_samples; i++) {
     UTI_AdjustTimeval(&samples[i].when, cooked, &samples[i].when, &delta_time,
         dfreq, doffset);
diff --git a/ntp_sources.c b/ntp_sources.c
index 3fa5b29..87e2e21 100644
--- a/ntp_sources.c
+++ b/ntp_sources.c
@@ -491,7 +491,11 @@ slew_sources(struct timeval *raw,
           UTI_IPToString(&records[i].remote_addr->ip_addr), dfreq, doffset);
 #endif
 
-      NCR_SlewTimes(records[i].data, cooked, dfreq, doffset);
+      if (change_type == LCL_ChangeUnknownStep) {
+        NCR_ResetInstance(records[i].data);
+      } else {
+        NCR_SlewTimes(records[i].data, cooked, dfreq, doffset);
+      }
     }
   }
 
diff --git a/refclock.c b/refclock.c
index 83b9488..d94c73c 100644
--- a/refclock.c
+++ b/refclock.c
@@ -581,8 +581,12 @@ slew_samples(struct timeval *raw, struct timeval *cooked, double dfreq,
 {
   int i;
 
-  for (i = 0; i < n_sources; i++)
-    filter_slew_samples(&refclocks[i].filter, cooked, dfreq, doffset);
+  for (i = 0; i < n_sources; i++) {
+    if (change_type == LCL_ChangeUnknownStep)
+      filter_reset(&refclocks[i].filter);
+    else
+      filter_slew_samples(&refclocks[i].filter, cooked, dfreq, doffset);
+  }
 }
 
 static void
diff --git a/reference.c b/reference.c
index 6038ce3..503f672 100644
--- a/reference.c
+++ b/reference.c
@@ -147,7 +147,10 @@ handle_slew(struct timeval *raw,
             LCL_ChangeType change_type,
             void *anything)
 {
-  if (change_type == LCL_ChangeStep) {
+  if (change_type == LCL_ChangeUnknownStep) {
+    last_ref_update.tv_sec = 0;
+    last_ref_update.tv_usec = 0;
+  } else if (change_type == LCL_ChangeStep) {
     UTI_AddDoubleToTimeval(&last_ref_update, -doffset, &last_ref_update);
   }
 }
diff --git a/rtc_linux.c b/rtc_linux.c
index 602345c..88c0160 100644
--- a/rtc_linux.c
+++ b/rtc_linux.c
@@ -273,6 +273,11 @@ slew_samples
   double delta_time;
   double old_seconds_fast, old_gain_rate;
 
+  if (change_type == LCL_ChangeUnknownStep) {
+    /* Drop all samples. */
+    n_samples = 0;
+  }
+
   for (i=0; i<n_samples; i++) {
     UTI_AdjustTimeval(system_times + i, cooked, system_times + i, &delta_time,
         dfreq, doffset);
diff --git a/sources.c b/sources.c
index d0600cc..6ad6bf2 100644
--- a/sources.c
+++ b/sources.c
@@ -1093,9 +1093,17 @@ slew_sources(struct timeval *raw,
   int i;
 
   for (i=0; i<n_sources; i++) {
-    SST_SlewSamples(sources[i]->stats, cooked, dfreq, doffset);
+    if (change_type == LCL_ChangeUnknownStep) {
+      SST_ResetInstance(sources[i]->stats);
+    } else {
+      SST_SlewSamples(sources[i]->stats, cooked, dfreq, doffset);
+    }
+  }
+
+  if (change_type == LCL_ChangeUnknownStep) {
+    /* After resetting no source is selectable, set reference unsynchronised */
+    SRC_SelectSource(NULL);
   }
-  
 }
 
 /* ================================================== */
diff --git a/sys_generic.c b/sys_generic.c
index 51c6824..2adbbfe 100644
--- a/sys_generic.c
+++ b/sys_generic.c
@@ -87,6 +87,7 @@ static double slew_error;
 /* ================================================== */
 
 static void handle_end_of_slew(void *anything);
+static void update_slew(void);
 
 /* ================================================== */
 /* Adjust slew_start on clock step */
@@ -95,8 +96,14 @@ static void
 handle_step(struct timeval *raw, struct timeval *cooked, double dfreq,
             double doffset, LCL_ChangeType change_type, void *anything)
 {
-  if (change_type == LCL_ChangeStep)
+  if (change_type == LCL_ChangeUnknownStep) {
+    /* Reset offset and slewing */
+    slew_start = *raw;
+    offset_register = 0.0;
+    update_slew();
+  } else if (change_type == LCL_ChangeStep) {
     UTI_AddDoubleToTimeval(&slew_start, -doffset, &slew_start);
+  }
 }
 
 /* ================================================== */
diff --git a/test/simulation/005-externalstep b/test/simulation/005-externalstep
index 3f26324..1e5527d 100755
--- a/test/simulation/005-externalstep
+++ b/test/simulation/005-externalstep
@@ -4,8 +4,8 @@
 
 test_start "external time step"
 
-min_sync_time=1300
-max_sync_time=1500
+min_sync_time=1500
+max_sync_time=1550
 
 for step in -1e2 1e2; do
 	# Make one step in 150th second