Which of the following synchronizes WSJT-X digital mode transmit/receive timing?

WSJT-X digital modes, such as FT8 and JT65, rely on strict time-division multiplexing where transmit and receive windows occur at exact, predetermined intervals. For example, FT8 uses strict 15-second cycles. To ensure stations decode each other properly, their computers must agree on exactly when a minute begins. Therefore, precise synchronization of computer clocks (usually via internet NTP servers or GPS time sources) is absolutely required for the software to function. Unlike traditional packet or RTTY, there are no sync-pulses or sync-fields transmitted over the air to establish timing between the two stations. The software relies entirely on the local PC's system clock being accurate to within a fraction of a second.