Although this eight-minute video's title is a misnomer---NTP isn't an <i>obscure</i> system, in that it is incredibly well-documented---it is still a reasonably informative and entertaining explainer. <media href="https://www.youtube.com/watch?v=CwZW0CO7F-g" src="https://www.youtube.com/v/CwZW0CO7F-g" source="YouTube" width="560px" author="Half as Interesting" caption="The Obscure System That Syncs All The World’s Clocks"> The system is called NTP---the Network Time Protocol---and comprises four tiers. <ul><b>Tier 0</b> is Atomic clocks, which measures the resonant frequency of Cesium atoms to obtain a regular "ticking" from nature itself. These are attached to servers in <b>Stratum 1</b>, usually a machine that is on-site. These are, in turn, attached to <b>Stratum 2</b> servers, things like <c>time.windows.com</c> or <c>pool.ntp.org</c>. Any machines that we use are almost certainly in <b>Stratum 3</b>, which are connected to <i>Stratum 2</i> machines.</ul> The machines coordinate between layers by relying primarily on their local clocks (usually kept running run by a CMOS battery on what passes for a motherboard) and re-synchronizing occasionally by "pinging" the layer above. They account for lag by including time sent and time received in messages, so that the sending system has four times with which to calculate the current time.