There are many NTP server implementations now. Here are some:

• ntpd
• chrony
• ntpsec
• openntpd
• sntp clients
  • systemd-timesyncd
  • sntp
• also:
  • ntimed – appears abandoned

Resources:

• Securing Network Time
  • A security review of three NTP implementations
  • concludes chrony is probably the best choice of ntpd, ntpsec, and chrony
• A rift in the NTP world
• Chrony: Comparison of NTP implementations
• NTPSec: Differences from NTP Classic
• NTPsec is not quite a full rewrite

I plan to give chrony a try.