Clock Generation with jitter

System Verilog Code:-

//---------------------------------------------------------------------------------------
// Name: jitter_clock_gen
// Author: Bharath Kumar Y
// Description: This module will generate a clock with
//              period                = 16ps
//              Max Skew/half clock   = +/-1ps
//              Max Skew/clock period = +/-2ps
// Copyright (C) Bharath Kumar Y
//----------------------------------------------------------------------------------------

`timescale 1ps/1ps

module jitter_clock_gen;
//Initializations
  bit req0=0;
  bit clk=0;
  integer seed;
  real zero_time=0;
  real one_time=8;


// Just to run the sim for some good amount of time to get
// enough clock edges
initial begin
  # 100 $finish;
end

// Actual clock generation clock
always
  #(8+$dist_uniform(seed,-1,1)) clk = ~clk;

// Checking of clock geneation implemented using
// Immediate assertions
// This block will assert errors when the
// half clock period != 8
always @* 
begin
  if(clk==0) begin
    req0=1;
    zero_time = $realtime;
    $display("zero_time = %t",$realtime);
    assert(!(($realtime<(one_time+8))));
    assert(!(($realtime>(one_time+8))));
  end

  if (req0==1 && clk==1) begin
    req0=0;
    one_time=$realtime;
    $display("time:%t:-req0=%b,req=%b",$realtime,req0,clk);
    assert(!(($realtime<(zero_time+8))));
    assert(!(($realtime>(zero_time+8))));
  end
end

endmodule

Output for the same code using ncsim:-

ncsim> run
zero_time =                   15
ncsim: *E,ASRTST (./fun1.sv,41): (time 15 PS) Assertion jitter_clock_gen.__assert_1 has failed
time:                  22:-req0=0,req=1
ncsim: *E,ASRTST (./fun1.sv,49): (time 22 PS) Assertion jitter_clock_gen.__assert_3 has failed
zero_time =                   29
ncsim: *E,ASRTST (./fun1.sv,41): (time 29 PS) Assertion jitter_clock_gen.__assert_1 has failed
time:                  37:-req0=0,req=1
zero_time =                   46
ncsim: *E,ASRTST (./fun1.sv,42): (time 46 PS) Assertion jitter_clock_gen.__assert_2 has failed
time:                  53:-req0=0,req=1
ncsim: *E,ASRTST (./fun1.sv,49): (time 53 PS) Assertion jitter_clock_gen.__assert_3 has failed
zero_time =                   60
ncsim: *E,ASRTST (./fun1.sv,41): (time 60 PS) Assertion jitter_clock_gen.__assert_1 has failed
time:                  68:-req0=0,req=1
zero_time =                   76
time:                  85:-req0=0,req=1
ncsim: *E,ASRTST (./fun1.sv,50): (time 85 PS) Assertion jitter_clock_gen.__assert_4 has failed
zero_time =                   93
Simulation complete via $finish(1) at time 100 PS + 0
./fun1.sv:26   # 100 $finish;