NTP servers always support both NTP and SNTP clients. The difference is one of synchronization accuracy, not communications compatibility. Although in theory both NTP and SNTP clients could run at the same time on a client this is not recommended. The SNTP updates will simply degrade the time accuracy. When other time protocol clients, such as IRIG, are also being used these clients must be coordinated with the NTP client to avoid synchronization problems.
RFC-1305 includes specifications for management of intermittent access to the NTP servers, broken servers, etc. The NTP servers do not need to be present and operational when the NTP process begins. NTP supports the use of multiple servers to provide backup and better accuracy. RFC-1305 specifies the mechanisms used by the NTP client. The site www.ntp.org provides extensive guidance and references regarding the most effective configurations for backups and multiple server configurations.
The local battery clock and client operating system must be properly UTC aware. NTP synchronization is in UTC. This can be a source of confusion because some computers are configured with their hardware clocks set to local time and the operating system set (incorrectly) to UTC. This is a common error that only becomes apparent when the devices attempt to synchronize clocks.