A major trend in computer architecture is multi-core processors. To fully exploit this type of parallel processor chip, programs running on it will have to be parallel as well. This means that even hard real-time embedded systems will be parallel. Therefore, it is of utmost importance that methods to analyze the timing properties of parallel real-time systems are developed. This paper presents an algorithm that is founded on abstract interpretation and derives safe approximations of the execution times of parallel programs. The algorithm is formulated and proven correct for a simple parallel language with parallel threads, shared memory and synchronization via locks.