Reliability estimation of High Performance Computing (HPC) systems enables resource allocation, and fault tolerance frameworks to minimize the performance loss due to unexpected failures. Recent studies have shown that compute nodes in HPC systems follow a time varying failure rate distribution such as Weibull, instead of the exponential distribution. In this paper, we propose a model for the Time to Failure (TTF) distribution of a system of k s-independent nodes when individual nodes exhibit time varying failure rates. We also present the system reliability, failure rates, Mean Time to Failure (MTTF), and derivations of the proposed system TTF model. The model is validated using observed data on time to failure.