Make a clone of the widget, modify it, then put your own widget in the dashboard. Widgets can be dealt with in Dashboards, the Widgets tab. This particular widget in the default Dashboard is Who's Online widget.
That widget uses a file, manager/controllers/default/dashboard/widget.grid-online.php. The line you're interested in is line 12,
$timetocheck = (time()-(60*20))+$this->modx->getOption('server_offset_time',null,0);
[ed. note: sottwell last edited this post 11 years, 9 months ago.]