Fecha de JavaScript menos segundos
// datenow minus 20 seconds
new Date(Date.now() - 20000);
Ham-Solo
// datenow minus 20 seconds
new Date(Date.now() - 20000);var endDate = somedate;
var startdate = new Date(endDate);
var durationInMinutes = 20;
startdate.setMinutes(endDate.getMinutes() - durationInMinutes);