Nick,
If you play this file back in Spike does Spike show the correct time? You can see the time displayed for each sweep right under the playback scrubber bar.
Yes, I have verified the timestamp is being set properly in the latest version of Spike (3.3.0). I have lots of files from previous versions that all appear to have valid time stamps.
I suspect your python parser is either reading this value incorrectly or is aligned improperly?
The time is in milliseconds since epoch where epoch is defined as 1-1-1970. Here is a website that shows the current millis since epoch. https://currentmillis.com/
Regards,
Andrew