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In this paper, we describe an algorithm built on top of a precision time protocol (PTP) implementation that allows for synchronization, syntonization and absolute time referencing to coordinated universal time (UTC), including the estimation of timestamp uncertainty. We call this the software grandmaster (SWGM) algorithm, since it provides similar services to a PTP grandmaster clock. We show that SWGM allows timestamps to be coordinated between multiple participants in a distributed measurement system with typical performance of plusmn 86 ns [root mean square (rms)] over commodity switched Ethernet connections using hardware PTP and hardware-derived timestamps. We further show that when software-derived timestamps are used the uncertainty in the timestamps is primarily driven by the latency of the system calls to read the PTP hardware, and may be on the order of 15-25 s (rms) depending on process priority, hardware bus speed, and host processor clock rate. We also show that SWGM is robust against dropped network packets up to approximately 60% loss of packets.