The Idea: In 2020, Peer Richelsen was running LeanHire, a contractor-hiring platform where Calendly was the default scheduling layer. It drove him mad - closed source, locked down, impossible to customise, and in his words, the opposite of what the rest of the modern dev stack had become. He spent weeks hunting for an open-source alternative. None existed. So he specced out his own version on a simple website, posted it to a waitlist, and built the first rough cut of the product. Then LeanHire got acquired. Peer took a full-time job as Head of Product at On Deck and paused the side project - but before shelving it, he emailed the waitlist looking for someone to take it over. A 17-year-old self-taught developer from the UK named Bailey Pumfleet replied within hours. Peer paid him $5,000 for two weeks of work. Two weeks became a co-founder partnership. This was the beginning of Cal.com.

The Execution:

• January 2021: Peer and Bailey officially co-found the project under the name Calendso, working remotely across Germany and the UK with zero outside funding. Bailey hard-codes the entire MVP in three weeks, with time zones temporarily locked to London because they haven't cracked the timezone logic yet.
• April 2021: Calendso launches on Product Hunt and hits #1 Product of the Day, Week, and Month - a clean sweep almost nobody pulls off. They close around 50 paying customers on day one despite having no free plan. The waitlist, built months before a single line of code, pays off instantly.
• September 2021: They rebrand from Calendso to Cal.com after buying the three-letter premium domain for a reported six figures, and ship their 1.0 launch. The name upgrade signals the bigger ambition - not a Calendly clone, but open scheduling infrastructure for the internet.
• December 2021: Cal.com raises a $7.4M seed round led by an individual investor with participation from On Deck, Remote First Capital, and Tribe Capital. Bailey turns 18 a few months later, making him one of the youngest co-founders to close a seed round of that size.
• April 2022: Four months after seed, they close a $25M Series A led by Alexis Ohanian's 776, with Obvious Ventures and OSS Capital piling in. Total raised crosses $32M. Users grow from roughly 10,000 to over 100,000 in 18 months.
• 2024-2025: Cal.com expands from scheduling into a full infrastructure play, launching Cal.ai for automated scheduling, white-label and embedded products for big platforms, and a run at becoming the maintainer of the world's largest Next.js open-source project.
• April 2026: After five years of being the poster child for open source, Cal.com announces it's going closed source, citing AI-powered security threats as the reason. A bold reversal, a split community, and Cal.diy spun off as the open-source descendant. The bet: proprietary beats transparent when AI makes every open codebase a honeypot.

The lesson? Two things. First: the fastest way to find a co-founder isn't LinkedIn - it's shipping something half-built, putting it in front of users, and seeing who in the waitlist replies "I can build that." Bailey became a primary co-founder because he did the work before being asked. Second: the moat isn't always the code. Cal.com spent five years winning on open source, then pivoted to closed source the moment the security calculus shifted. The best founders don't marry their original thesis - they update it when the world changes.