Time can be displayed during simulation using %t.

$display("%t", $realtime);

Timeformat is used to control the way (%t) this is displayed:

//$timeformat(unit#, prec#, "unit", minwidth);
$timeformat(-3, 2, " ms", 10); // -3 and " ms" give useful display msg
  1. unit is the base that time is to be displayed in, from 0 to -15
  2. precision is the number of decimal points to display.
  3. “unit” is a string appended to the time, such as “ ns”.

  4. minwidth is the minimum number of characters that will be displayed.

    0 = 1 sec -1 = 100 ms -2 = 10 ms -3 = 1 ms -4 = 100 us -5 = 10 us -6 = 1 us -7 = 100 ns -8 = 10 ns -9 = 1 ns -10 = 100 ps -11 = 10 ps -12 = 1 ps -13 = 100 fs -14 = 10 fs -15 = 1 fs