An o-line scheduling algorithm considers resource, precedence, and synchronisation requirements of a task graph, and generates a schedule guaranteeing its timing requirements. This schedule must, however, be executed in adynamicandunpredictableoperatingenvironmentwhereresourcesmayfail and tasks may execute longer than expected. To accommodate such executionuncertainties, thispaperaddressesthesynthesisofrobusttaskschedules usingaslack-basedapproachandproposesasolutionusingintegerlinearprogramming (ILP). Earlier we formulated a time slot based ILP model whose solutions maximise the temporal exibility of the overall task schedule. In this paper, we propose an improved, interval based model, compare it to the former, and evaluate both on a set of random scenarios using two public domain ILP solvers and a proprietary SAT/ILP mixed solver.