Why WSJT-X modes such as FT8, FT4, JT65 and WSPR use fixed transmit/receive sequences keyed to UTC, for example FT8's 15-second slots that begin at 0, 15, 30 and 45 seconds past the minute. The decoder expects the signal to start at a known instant, so each station's computer clock must be set accurately, typically within about a second, using an internet time service (NTP) or GPS. If your clock drifts, your transmissions land outside the slot and other stations cannot decode you.
Watch out Sync fields or sync pulses embedded in the transmission are how many other digital protocols, like packet or PACTOR, establish timing, but WSJT-X relies on external clock accuracy instead of an in-signal sync marker.
FT8 lives on the clock: keep your PC within a second of UTC or nobody decodes you.