Because of the fast pace of the information technology, an electronic product may be in field for just a couple of years. Therefore, the instantaneous availability and interval availability during its operational lifetime are more important metrics than the steady-state availability. This paper presents a stair-step approximation to the instantaneous availability and interval availability for the systems with time-varying failure rate. The precision of the stair-step approximation is discussed. Numerical results for the systems with Weibull failure rate reveal that the steady-state availability may give an optimistic (pessimistic) evaluation to the systems with decreasing (increasing) failure rate.