Early Macs normally had their clocks synced manually, until System 8.5 introduced support for NTP. That later switched to a proprietary service, timed, in macOS 10.13 High Sierra.
ntpd
High Sierra has done away with the ntpd daemon for keeping your Mac’s clock in sync with a standard time server. This now works as in iOS, using the timed daemon.
The Eclectic Light Company 