Poul-Henning Kamp has shown that you can do this with NTP, with source code that actually follows the standard well. PTP makes a great refclock for more typical NTP implementations, but is not a replacement.

I haven’t played around much with chrony and chronyd, so I would be hesitant to recommend it over anything else. I know of some other software I have experience with that I would recommend against, but I don’t want to start any political wars here.