Re-read: "what is the maximum possible difference in local UTC times at the sync moment?" — likely means: over possible sync events, what's the largest instant-to-instant local UTC difference when syncing? - Groen Casting

Understanding the Context

Local time zones are defined by their offset from Universal Time (UT), which aligns closely with Coordinated Universal Time (UTC). The maximum offset between any time zone and UTC is approximately ±14 hours and 26 minutes, corresponding to the difference between the extreme eastern and western extremes of longitude—essentially the span from UTC−12:00 in the eastern Pacific to UTC+14:00 across the International Date Line. Since UTC itself is a time standard without daylight saving adjustments, this full range applies globally.

The Role of Synchronization in Minimizing Differences

Modern synchronization protocols strive to keep local clocks within milliseconds of UTC. Systems like NTP use hierarchical server hierarchies and feedback mechanisms to correct clock drift, reducing offset to within a few milliseconds under ideal network conditions. However, during the critical sync moment—when a device receives or sends a time reference—the physical latency of signal transmission, dependent on network distance and propagation speed, introduces a time delay that directly impacts the measured sync discrepancy.

Maximum Instantaneous UTC Difference at the Sync Moment

Key Insights

The instantaneous difference in local UTC times at the synchronization “sync moment” hinges on two primary factors:

1. Network Propagation Delay:
   Digital time signals travel at or near the speed of light (~300,000 km/s through fiber or ~299,700 km/s over fiber). For a signal sent from a reference server to a distant client, the maximum one-way propagation delay over roughly 20,000 km (a typical intercontinental connection) is about 66.7 milliseconds (66,700 μs). Thus, a time reference sent at UTC 12:00:00.000 UTC might arrive locally at 12:00:00.066 UTC—creating a local UTC offset equivalent of +66.7 ms.

Conversely, if a local time update is broadcast globally (e.g., server pushing time), the active clock composing the message will experience its own offset relative to UTC, potentially inward toward the negative side (older UTC boundaries). The combined effect during sync is bounded by the maximum physical distance between nodes.

2. Signal Latency vs. Synchronization Window:
   Precise synchronization typically relies on echo-location (sending a signal and waiting for response) or reference message models where the time to round-trip (RT) is known and compensated. Even then, during the decisive sync instant—say, a message timestamp meant to synchronize clocks—the printed or logged time reflects the local endpoint’s time relative to UTC at dispatch, not real-time UTC. This creates a one-shot offset bounded by network speed.

Theoretical Maximum Instantaneous Difference

Final Thoughts

At the moment a sync event occurs—such as a time protocol message being sent or received—the largest possible difference in local UTC times reflects the practical propagation delay across the farthest possible network path. Under worst-case latency:

• A client in Sydney syncing with a server in Honolulu experiences a one-way delay of ~148 ms (round-trip ~148 ms), effectively shifting local UTC reference by ±66.7 ms relative to the sender’s timestamp.
  
• Similarly, intercontinental communications exceeding typical 170,000 km distances can register up to ~566 ms (2×80,000 km), yielding a theoretical UTC offset difference of about ±–5.9 minutes (±356 ms).

However, instant-to-instant local UTC difference at the sync moment—the raw offset measured at the exact sync timestamp—is fundamentally constrained by network physics, not clock wall-clock drift. Thus, the maximum instantaneous difference is effectively the maximum one-way propagation delay experienced across global infrastructure at sync timing, roughly ±66.7 ms for transoceanic distances, or up to 180 ms considering worst-case round-trip latency in conservative network models.

Practical Implications and best Practices

• Time calibration systems account for propagation delays by embedding latency corrections into time metadata.
  
• GPS-based synchronization leverages satellite signals with ultra-precise timing but still requires localization-based latency compensation.
  
• Distributed systems often use software-based time adjustment algorithms to mitigate sync moment disparities caused by propagation lag.

Conclusion