UML is the standard visual object modeling language which may be very useful as a system design communication language. However, UML as a real-time modeling language has limitations. It basically provides a lot of syntax, but not enough semantics. The UML Profile for Schedulability, Performance and Time, extends the notation and semantics for the real-time domain. It is supposed to overcome the limitations which are related to and provide suitability for modeling real-time systems. Since we are mainly concerned with the problem of real-time operating system scheduling, the article discusses the ability of UML and its profile to determine the schedulability of a planned piece of software.