Check the accuracy of your computer's clock. Found via https://lists.dyne.org/lurker/message/20210621.225323.41fc8a46.en.html, where the following recipe to adjust the time on a unix system is presented:
 date --rfc-3339=ns && \
 date --rfc-3339=ns -s '+0.096 seconds' && \
 date --rfc-3339=ns
where the string after -s shall be replaced by the report from time.is

A Simple PICTIC using a 10 MHz XO timebase, no prescaler, and a unity gain buffer provides an interpolator gain of 400 for a resolution of 250ps suitable for GPS monitoring.

clockspeed uses a hardware tick counter to compensate for a persistently fast or slow system clock. Given a few time measurements from a reliable source, it computes and then eliminates the clock skew.

sntpclock checks another system's NTP clock, and prints the results in a format suitable for input to clockspeed. sntpclock is the simplest available NTP/SNTP client.

taiclock and taiclockd form an even simpler alternative to SNTP. They are suitable for precise time synchronization over a local area network, without the hassles and potential security problems of an NTP server.

An experimental implementation of tlsdate (https://github.com/ioerror/tlsdate) in Go.
WARNING: This software has not been reviewed. You probably shouldn't use it.

The TAPR TICC is a two-channel timestamping counter with better than 60 picosecond resolution and less than 100 picosecond typical jitter. It has an Allan Deviation noise floor below 1x10-10 for a one second measurement.

The entire desire to do this came about from reading about building a Stratum 1 clock using a Raspberry PI at http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html and from the use of a Sony radio clock to provide WWVB pulses from the WWVB receiver module that was used in the radio clock.

However, all of the projects and examples I found used Linux which I did not really want to use. I much preferred to use NetBSD, but it lacked a number of critical features that would be required to make a properly working Stratum 1 clock. Interrupt support for the GPIO pins was needed and adding a PPSAPI driver that could be triggered by a GPIO interrupt would be necessary.

WaveDrom draws your Timing Diagram or Waveform from simple textual description.
It comes with description language, rendering engine and the editor.
WaveDrom editor works in the browser or can be installed on your system.
Rendering engine can be embeded into any webpage.

Leap second observations, long wave receivers for NTP.

White Rabbit provides sub-nanosecond accuracy and picoseconds precision of synchronization for large distributed systems. It also allows for deterministic and reliable data delivery.
White Rabbit allows you to precision time-tag measured data and lets you trigger data taking in large installations while at the same time using the same network to transmit data.

   sub-nanosecond synchronization
   connecting thousands of nodes
   typical distances of 10 km between nodes
   Ethernet-based gigabit rate reliable data transfer
   fully open hardware, firmware and software
   multi-vendor commercially produced hardware

Blog describing the development of a software library for DFC77 receivers.