Hi @dheup,
I think the answer to this could vary depending on whether your school/institution (or course) has enabled the Assignment Enhancements/Redesign feature or not.
My institution has that enabled, and within the submission workflow there, a file has to be uploaded first before a student hits the submit button, so the time is exactly when the student hits the submit button. I looked at the guide for the original Assignments interface to refresh my memory, and I do believe it's the same scenario I just described for the redesigned assignments interface too, but I seem to recall that the submit button may have been clickable there before a file was done uploading. In that scenario, I think the submission would show an error for the student. It's been a number of years since we've used that interface at my institution though, so the situation may have changed slightly since then.
Overall, I think the intention is that files have to be uploaded before a submission would be generated, and the time shows them the submission happened and would therefore have to be after the files were uploaded.
Hope this helps!
-Chris