NTP on FreeBSD 12.1

Add the following to /etc/ntp.conf

pool ntp2.aliyun.com iburst
pool 1.tw.pool.ntp.org iburst
pool 0.freebsd.pool.ntp.org iburst
server ntp1.aliyun.com iburst

Add the following to /etc/rc.conf

ntpd_enable="YES"
ntpd_sync_on_start="YES"

Set ntpd_enable="YES" to start ntpd at boot time. Once ntpd_enable=YES has been added to /etc/rc.conf.
Set ntpd_sync_on_start="YES" to allow ntpd to step the clock any amount, one time at startup.
Start NTP:

service ntpd start

Reference:

Clock Synchronization with NTP