Preprint. Uncertain operation durations can turn apparently efficient surgery schedules into long delays and overtime. This study separates uncertainty estimation from schedule optimization and makes the reliability level of each surgery a scheduling decision. A buffer engine supplies duration estimates for different reliability levels, while the model jointly chooses assignments, order and start times. A rolling-origin case study using scheduling instances from HLA Moncloa Hospital in Madrid reports fewer long delays and less overtime than a deterministic mean-duration baseline. The results concern operational scheduling rather than individual clinical decisions.