Emerging applications of Ultra-Wide Band (UWB) combine low to medium rate communications with positioning capabilities allowing centimeter level accuracy in ranging. For positioning systems employing UWB radios, time-based schemes provide very good accuracy due to the high time resolution of UWB signals. These time-based positioning systems rely on measurements of travel times of signal between nodes allowing to estimate the distance between nodes. The standard IEEE 802.15.4a-2007 propose TWR and SDS-TWR time-based protocols for ranging purpose. However, the accuracy of TWR is quite poor due to the effects of clock skews. SDS-TWR mitigates the clock skew error at the expenses of the number of message exchanges, which is increased. In this work, we present a novel approach for accurately estimating the ToF in UWB taking into account the clock skew between nodes while minimising the number of exchanged messages. Experimentations were carried out in our Open Source Framework, which enables fast prototyping of protocols based on an UWB Physical Layer.