A space-separated list of NTP server host names or IP addresses. During runtime this list is combined with any per-interface NTP servers acquired from systemd-networkd.service(8). systemd-timesyncd will contact all configured system or per-interface servers in turn, until one responds. When the empty string is assigned, the list of NTP servers is reset, and all prior assignments will have no effect. Rationale: Time synchronization is important to support time sensitive security mechanisms like Kerberos and also ensures log files have consistent time records across the enterprise, which aids in forensic investigations.