scheduleOneTime.cls

//This can be any time in future
Datetime nextRun = System.now().addHours(2);

//Generating cron expression for next run time
String year = String.valueOf(nextRun.year());
String day = String.valueOf(nextRun.day());
String hour = String.valueOf(nextRun.hour());
String min = String.valueOf(nextRun.minute());
String sec = String.valueOf(nextRun.second());
String cronExp = sec + ' ' + min + ' ' + hour + ' '+day+' '+nextRun.format('MMM').toUpperCase()+' ? '+year;

//Scheduling at the specified time
TrackerSchedulable trackerJob = new TrackerSchedulable();
System.schedule('Tracker Job', cronExp, trackerJob);