In this paper, we propose a method of improving the accuracy of NTP time synchronization by taking into account asymmetric transmission delays due to different bandwidth or routing on the forward and backward paths. The method consists of calibrating NTP synchronization by: i) deploying a time box with a GPS clock at a given client, ii) measuring the one-way transmission delay on the forward and backward path and finding the minimal delays, iii) using the minimal delays in the estimation of the clock offset at the client to take into account path asymmetry, and iv) recalibrating if routes change. The paper first reports on the measurements of one-way transmission delays between a client and a server interconnected by several routers. We then use the parameters of the delay distributions to estimate the clock offset at the client. To validate the proposed method, we have compared the clock offsets computed by standard NTP and calibrated NTP based on the GPS time reference. The measurements show significant improvement of the NTP time synchronization accuracy and precision.