The question asks: "what is the maximum possible difference in local UTC times at the sync moment?" — this refers to how large the time gap *can be* while still synchronizing. - Tacotoon

Understanding the Context

UTC, or Coordinated Universal Time, is the world’s primary time standard based on International Atomic Time (TAI) with leap seconds for precision. Local time zones offset from UTC, resulting in varying local times across regions. When devices sync their clocks—via protocols like NTP (Network Time Protocol)—they adjust to UTC to ensure consistency. But the synchronization process itself introduces a momentary time gap.

The Sync Moment: How Time Difference is Measured

During synchronization, each local clock checks its current offset from UTC. The maximum possible difference at the sync moment refers to the largest expected discrepancy between the local time on a device and the standardized UTC exactly when resync occurs. This difference determines whether resync maintains or corrects the clock’s accuracy.

Theoretical and Practical Limits of Time Differences

Key Insights

1. NTP Resynchronization Basics:
   When a device syncs with an NTP server, it measures the current time offset—including any drift due to GPU delays, network latency, and clock strout. The protocol computes the offset and adjusts the local clock accordingly. However, residual error usually exists.

2. Maximum Theoretical Offset:
   The maximum possible difference during sync depends on system clock precision and network conditions. In ideal conditions (perfect synchronization), offset is zero. In practice, due to hardware and network variability, NTP achieves sub-millisecond accuracy. But theoretically, without network interruptions, the clock offset at moment of sync could be as large as:

• Up to ±100 milliseconds (0.1 seconds): Minor discrepancies from drift or latency may cause local clocks to differ by this range.
  
  • Theoretical upper bound under worst-case conditions: Up to ±150 ms (0.15 seconds) might occur due to combined clock jitter, latency spikes, and imperfect offset calculation—though modern systems typically handle well below this.

3. Regulatory and Protocol Constraints:
   ISO 57000 and NTP standards specify precision requirements. While UTC itself does not permit mismatches, the sync moment gap must be minor enough to prevent desynchronization. If local UTC clocks differ by more than ~100 ms at sync, resynchronization would fail or require extended adjustment, breaking time integrity.

Real-World Implications

Final Thoughts

• Internet Time Protocols (NTP, PTP): These systems account for network latency and clock stability. A sync check often includes estimating drift, ensuring the difference remains within safe bounds.
  
• Cryptography and Logging: High-precision systems, such as financial services or scientific networks, require sync offsets below 100 ms to prevent data inconsistency.
  
• Consumer Devices: Smartphones, computers, and IoT devices aim for ±50 ms sync gaps; beyond this, manual reset or extended correction is often needed.

Conclusion: Best Practices for Tight Synchronization

To minimize time differences during sync, ensure stable networks, fresh time sources (e.g., GPS), and protocols with redundancy. Understanding the maximum acceptable jucture—typically under 100–150 ms—helps design resilient time systems that maintain UTC alignment reliably.

Summary:
The maximum possible difference in local UTC times at the synchronization moment is typically around ±100 to ±150 milliseconds, depending on system precision, network conditions, and protocol implementation. While UTC maintains global consistency, local discrepancies during sync are inevitable; modern timekeeping systems aim to keep these gaps minimal to ensure continuous accurate alignment. For mission-critical applications, adherence to strict sync protocols ensures the time difference remains within safe operational limits.