I agree with dbmag9 that your university's regulations are definitive. If they do not give any guidance - and if you have nothing helpful from your supervisor, department, etc. - then I would suggest the following: The date on the thesis should be the date when the text was most recently submitted for examination.
Typically, a thesis undergoes minor or major corrections before being accepted. However, the approval of corrections is done without repeating the full examination process. More significant changes require "revise and resubmit" and in my mind this justifies bumping the date. But for small changes, the text is essentially the same version. That said, I don't think it's at all wrong to use the final hardbound submission date. I wouldn't pick the viva date, even if you had no corrections: the text was produced for the viva, not at the viva.
In my case, I had to include the text "Submitted for the degree of Doctor of Philosophy" on the title page of the hardbound version, and I had to include a statement affirming that the text was the same as the original submitted thesis, aside from changes required by the examiners. This made me think that the accompanying date ought to be the date of the submission, not the date of the approval. (In fact, there were many different possible approval dates - examiners, the department, the central university authorities, the actual graduation ceremony. None of these seemed like the obvious correct choice.)