July 8, 2015 for me: www.biorxiv.org/content/10.1... . Although I go back to arXiv from Jan 24, 2007: arxiv.org/abs/q-bio/07... .
Following a discussion on preprints, I went looking for my oldest submission to @biorxivpreprint.bsky.social: April 2014, 5 months after bioRxiv started. www.biorxiv.org/content/10.1...