In what format is the offline_timestamp key in your personal data?

[Artaro]

In the extended personal data, there is a key called "offline_timestamp". In the ReadMeFirst it has the following description: "This field is a timestamp of when offline mode was used, if
used.". When looking at these values, there seems to be no correlation with the "ts" key and I can't seem to find any pattern that describes an actual timestamp.So is there logic behind these offline timestamps?