The oldest submission I can find using the search form is this one from the 8th of March 2006:

https://hwbot.org/submission/498730_trouffman_cpu_frequency_pentium_4_561_5263_mhz

It's hard to know whether this is really the first sub as the fact that past world records were added "posthumously" clouds the picture. HWBot's copyright is 2004 - present (as indicated by the page footer), does that mean the project was in startup mode 2004 - 2006 with no submissions made or have those subs made in that timeframe been purged somehow?

snippit from the wordpress archives. Only people that maybe could reply to that question are no longer active here

HWBOT started 2005 as a school task by Frederik Colardyn, but due to large success development continued after graduation...

 

List of teams as of July 7, 2005.

 

If you look at the number in the database 488270 It was presented earlier.

Number of results as of January 1, 2006 for processor tests

Before hwbot was hwbot, it was called "mbot". This was Frederik's school project. Forum users could report their overclocking result in a forum thread and the "mbot" would update the opening post with the rankings. It would also show the results from other forums.

The first Mbot submission should be in April 2004. Frederik used his "home" forum to set up the Mbot test thread http://www.madshrimps.be/vbulletin/f14/mbot-test-thread-4716/. Some forums still have the original thread up as well. For example the Portuguese forum Zwame still has the original forum thread with submissions from May 2004 https://forum.zwame.pt/threads/hwbot-3dmark01-ranking.36163/.

Frederik registered the hwbot.org domain name in June 2005 http://www.madshrimps.be/vbulletin/f10/mbot-hwbot-launches-hwbot-org-15601/. Key features of the first site were: browse results, watch how other teams are doing, and edit bot settings. Later on you could also create an HWBOT account and link your forum accounts to your hwbot account. Then you'd automatically have access to all the scores you posted in the different forums and update information.

One year in, the site had 2500 users and about 10000 results http://www.madshrimps.be/vbulletin/f10/mbot-hwbot-launches-hwbot-org-15601/#post127977

I don't remember exactly what happened in March 2006, but if I recall correctly HWBOT moved to a different database model and the original submission dates were not imported in the new database. All the scores submitted prior to this day would have the same date in the database. That's why you'll find a lot of old submissions around March. Submission IDs don't necessarily reflect their date of submission.

TLDR: around April 27, 2004