The "User/User" Reference
While it is normally not required, it is possible for you as the "User" to override the System Time (even if it is synchronized to a valid reference) with a manually set time, steered by an undisciplined oscillator, and use this manually set Time as an output reference. This concept is referred to as the User/User reference, because both the Time, and the 1PPS reference are not linked to any UTC-based external reference, but hand-set by you.
Caution: Since the User/User reference is not traceable to a valid reference, it does not qualify as a legitimate time source. Operating NetClock with a manually set User time bears the risk of inadvertently outputting an illegitimate System Time thought to be a valid reference time.
The User/User reference is provided for the following use cases:
- No external references are available (yet), but you need a reference for testing or setup purposes. This may be the case e.g., while waiting for a GNSS antenna to be installed.
- No external references are required e.g., if NetClock is used solely to synchronize computers on a network, with no need for traceable UTC-based timing.
- To utilize a backup reference as soon as possible after a power cycle or reboot of NetClock, while waiting for the primary reference (e.g., GNSS) to become valid. To this end, in the Edit System Time window, the checkbox Synchronize to Battery Backed Time on Startup must be checked, AND the User/User reference is assigned a reference priority number other than "1". Note that a Time jump and/or 1PPS jump are likely to happen once the primary reference becomes valid.
Combining a User Time reference with a non-User 1PPS reference or vice versa is not a typical use case. Use the Local System reference instead, see The "Local System" Reference.
In order to "validate" (= greenstatus lights) the User/User reference, the hand-set time must be manually submitted every time after NetClock reboots or resets, or after the Holdover period has expired: In the Edit System Time window, the checkbox Manual Time Set must be checked. The System Time displayed in the field below will become valid the moment the Submit button is clicked.
See also below, "How long will the User/User reference be valid?": The notion of limiting the validity of the User/User reference also serves as a safety feature.
Since the User/User reference does not qualify as a legitimate, traceable time, it becomes invalid once NetClock is reset, or power-cycles, or after the Holdover Time expires (whichever occurs first). It then needs to be set manually and submitted again (Edit System Time > Manual Time Set).
The only workaround for this is Using Battery Backed Time on Startup. This will allow NetClock to apply the User/User reference after a power-cycle without manual intervention.
Otherwise, the large time correction that needs to be bridged when switching from a lost reference to a valid reference, or from a valid reference to a higher-priority reference that has become available again, will cause NTP to exit synchronization. If the difference is under 1 second, NTP will remain in sync and will "slew" (over a period of time) to the new reference time.