Network flow measurement and analysis are important parts of network management and security. Flow data analysis is a challenging task which is often rendered harder by pitfalls in a monitoring pipeline. In this paper we focus on timestamps since many analysis procedures utilize timestamps to reveal various characteristics of network traffic. Unfortunately, the timestamps are not always that reliable as it may seem. We propose an algorithm to estimate the percentage of correctly assigned timestamps to flow records with respect to the sequence of a request and a response flow. We simulate various timestamp failures and we evaluate the failures using the proposed algorithm. We demonstrate the usage of the algorithm in the use case of bidirectional flow orientation.