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Computers are increasingly being applied in time and safety critical areas of our everyday life. Analysis of those computer systems, and prediction of their timing properties -- and related safety properties -- are still, in the majority of cases, made by using empirical methods like simulation and testing. The development of appropriate analytical methods is hindered by insufficient understanding of the essence and role of time in real-time systems. This paper makes an attempt to systematize the variety time-bound terms, understandings and theories used for describing, analysing and verifying the properties of real-time (embedded) software. Recommendations are made about the time concepts required for complete analysis of timing correctness of embedded real-time software. Time concepts usable in real-time software are projected to the background of those used in other fields of science and technology. This topic has not been to widely discussed earlier. Recently some interest in the essence of time as used in programming has been shown by Hoogeboom and Halang (1991), Motus (1992) and Motus and Rodd (1992).