WSJT-X DT values - What are these really representing?

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W7OWO
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WSJT-X DT values - What are these really representing?

For a while I've always wondered why my DT values are generally between +0.7 to +1.2 even though I use time synchronizing software. Over the weekend, reading over PA0O's and K4RCG's discussion on their higher than usual DT values on Chat, I decided to look into my issue a bit more.

I first verified my understanding by reading the documentation once again, and see my understanding that the numbers are supposed to represent "the signal’s time offset in seconds relative to your computer clock" was valid. I've tried both Dimension 4 and Meinberg and under both, the DT values are pretty much the same. Until now, I have always assumed my usual offset of nearly +1 second was related to some weirdness caused from my slow DSL internet, as I rarely get over 1.1 Mbps. So I finally decided to test that theory out by writing my on NTP client and validating what is really going on.

I set Meinberg synchronization to prefer a nearby Universities NTP server and built my program to use it as well, hoping the network path would be somewhat consistent. I then had my program read the computers time, then fetch the time from the NTP server and compute the difference to the nearest millisecond. Running this program where it queries the NTP server every 5 minutes, and averaging the time for the runs over two hours, if I throw away the very first call to the NTP server, I averaged about 0.022 seconds, with a range of about +- 0.005 seconds. I also timed the length it took to get the system time and the return from the NTP time and it was almost consistently .045 seconds.

This is a considerably shorter time than the .7 seconds or more I see consistently with my DT values. And I can't believe that that many folks are that consistently off from me. This brings me back to the title of this post. What are these DT values really representing?