Try this:

var t = new GlideDateTime();
var f = new GlideDateTime('2019-04-08 16:43:33');
var dur = GlideDateTime.subtract(t, f);

var duration = dur.getNumericValue();
var durationSeconds = (duration/1000);

gs.info(t);
gs.info(dur.getDisplayValue());
gs.info(durationSeconds );