This dissertation investigates the question: How do we precisely access and control time in a network of computer systems? Time is fundamental for network measurements. It is fundamental in measuring one-way delay and round trip times, which are important for network research, monitoring, and applications. Further, measuring such metrics requires precise timestamps, control of time gaps between messages and synchronized clocks. However, as the speed of computer networks increase and processing delays of network devices decrease, it is challenging to perform network measurements precisely. The key approach that this dissertation explores to controlling time and achieving precise network measurements is to use the physical layer of the network stack. It allows the exploitation of two observations: First, when two physical layers are connected via a cable, each physical layer always generates either data or special characters to maintain the link connectivity. Second, such continuous generation allows two physical layers to be synchronized for clock and bit recovery. As a result, the precision of timestamping can be improved by counting the number of special characters between messages in the physical layer. Further, the precision of pacing can be improved by controlling the number of special characters between messages in the physical layer. Moreover, the precision of synchronized clocks can be improved by running a protocol inside the physical layer by extending bit-level synchronization. Subsequently, we make three contributions embodied in the design, implementation and evaluation of our approaches. First, we present how to improve the precision of timestamping and pacing via access to the physical layer at sub-nanosecond scale. SoNIC implements the physical layer in software and allows users to control and access every bit in the physical layer. Second, we demonstrate that precise timestamping and pacing can improve the performance of network applications with two examples: Covert timing channels and available bandwidth estimation. A covert timing channel, Chupja, is high-bandwidth and robust and can deliver hidden messages while avoiding detection. An available bandwidth estimation algorithm, MinProbe, can accurately estimate the available bandwidth in a high-speed network. Finally, we present how to improve the precision of synchronized clocks via access to the physical layer. DTP, Datacenter Time Protocol, extends the physical layer's link-level synchronization and implements a peer-to-peer clock synchronization protocol with bounded nanosecond precision. Together, these systems and approaches represent important steps towards precise network measurements.