[assttoasstmgr]

Not just that, but PTP switches modify the frame header to take into account the delta of the propagation delay through the switch. Everything is synced to the GM clock which is typically sync'd to GPS and your GM clock can itself be designed to ensure stability (e.g. temperature controlled oscillator). Furthermore, if reading the incoming data using a specialized NIC (they do this in finance, high speed trading for example) the timestamps are decoded in hardware. All this is done at the layer 2 level.