This has been my precision time reference for years – Emerald Time Review

Fantastic simple, and highly useful. This gives you the extreme precision time of NTP, with lots of useful options. Specify servers, audible ticks/beeps, different display options, status of NTP time sync quality, etc. I often use it as a time sync reference on data logging videos- just point the camera at Emerald Time on my iPhone screen at the beginning of a video. Love this!!
Review by Steamfire on Emerald Time.