There is an increasing demand for methods that calculate the worst case execution time (WCET) of real time programs. The calculations are typically based on path information for the program, such as the maximum number of iterations in loops and identification of infeasible paths. Most often, this information is given as manual annotations by the programmer. Our method calculates path information automatically for real time programs, thereby relieving the programmer from tedious and error-prone work. The method, based on abstract interpretation, generates a safe approximation of the path information. A trade-off between quality and calculation cost is made, since finding the exact information is a complex, often intractable problem for nontrivial programs. We describe the method by a simple, worked example. We show that our prototype tool is capable of analyzing a number of program examples from the WCET literature, without using any extra information or consideration of special cases needed in other approaches.