diff --git a/PDP10/pdp10_tim.c b/PDP10/pdp10_tim.c
index 43bee3f0..c559a0ea 100644
--- a/PDP10/pdp10_tim.c
+++ b/PDP10/pdp10_tim.c
@@ -25,6 +25,7 @@
 
    tim          timer subsystem
 
+   10-Nov-16    ***     Fix wallclock issue for 50 Hz systems (R. Voorhorst)
    18-Apr-12    RMS     Removed absolute scheduling on reset
    18-Jun-07    RMS     Added UNIT_IDLE flag
    03-Nov-06    RMS     Rewritten to support idling
@@ -291,7 +292,7 @@ tim_new_period = new_interval & ~TIM_HWRE_MASK;
 if (new_interval & TIM_HWRE_MASK) tim_new_period += 010000;
     
 /* clk_tps is the new number of clocks ticks per second */
-clk_tps = (int32) ceil((double)TIM_HW_FREQ /(double)tim_new_period);
+clk_tps = (int32) round((double)TIM_HW_FREQ /(double)tim_new_period);
    
 /* tmxr is polled every tim_mult clks.  Compute the divisor matching the target. */
 tim_mult = (clk_tps <= TIM_TMXR_FREQ) ? 1 : (clk_tps / TIM_TMXR_FREQ) ;