All of the JT modes (JT65, JT9), and now the new mode FT8 are sensitive to correct system time within a couple of seconds. This is because the transmit/receive cycle starts and stops at a predetermined time, and the software of each transmitting station and each remote receiving station need to be in sync in order for each transmission to get successfully decoded.
FT8 seems to be forgiving within about 1.5 seconds of difference, but above this it flags the delta between stations in the DT column:
On a new laptop with Windows 10, even though the date and time is set to set automatically using a remote time server on the internet, almost every FT8 transmission received was consistenly > 1.5 seconds in difference.
To fix this I downloaded and installed Dimension 4 and set it to start automatically. Sync’d the time and now we’re all set – you can see the received calls below in the DT column are all now within a couple of 0.1s after the sync: