Duly filed at 23:56. Having assiduously filed papers through out the year, this year's tax return completion started at 23:42, which I think makes this year about as late a start as I can remember.
UPDATE: Soon after I got a reply by email: "Thank you for sending the Self Assessment submission online. The submission for reference 32*******202 was successfully received on 2010-01-31T23:56:51.676 and is being processed."
So they acknowledge the time in milliseconds, and I wasn't really cutting it that fine.
I could have filed up to 488,323 milliseconds later.