Regarding the syntax of $timeformat

What does minimum field width mean in $timeformat ?
I mean the 4th (last) argument.

The last argument specifies the minimum field width of time being printed. Here is the syntax for $timeformat:

$timeformat [ ( units_number , precision_number , suffix_string , minimum_field_width ) ] ;

Units number suggests the simulation precision time as specified by timescale.
Precision number is the number of integers displayed after decimal point.
Suffix string is the string suffixed at the end of time.
Minimum field width suggests the minimum number of characters used to display the time.

Lets see a simple example to understand the forth argument:

  initial begin
    string str;
    $timeformat(-9,3,"ns",5);
    #1;
    str = $sformatf("%t",$time);
    $display("%t-is",$time);
    $display("str len = %0d",str.len());
  end

  // Output
  1.000ns-is
  str len = 7

  initial begin
    string str;
    $timeformat(-9,3,"ns",20); // MINIMUM width is 20
    #1;
    str = $sformatf("%t",$time);
    $display("%t-is",$time);
    $display("str len = %0d",str.len());
  end

  // Output
               1.000ns-is
  str len = 20

The first sample shows that the width of time is 7 characters (“1.000ns”) this is exceeding the minimum width, so the actual width is taken. While in the second example, the minimum width is 20, so the string is assigned as 20 characters wide.

Refer IEEE 1800-2012 section 20.4.2 for more details.