calendly like free self hosted is one of the most honest queries in search. It names the shape of the thing wanted -- a link you send someone so they can pick a time -- and then names the two conditions: no monthly bill, and running on hardware the searcher controls. Those two conditions pull in different directions more often than the query suggests, and it is worth being precise about why.
Free and self-hosted are not the same promise
Free usually means one of three arrangements. A free tier of a hosted service, which is free until the feature you actually need sits one rung up. An open-source project you install yourself, which is free of license cost and not free of your time. Or a one-time purchase, which is not free at all but stops asking.
Self-hosted is a separate question: whose machine holds the data. A free hosted tier is not self-hosted, however little it costs. An open-source project you never manage to keep patched is self-hosted in name and somebody else's problem in practice.
The searcher behind this query is usually after the third arrangement with the second's control: a scheduling link on their own domain, in a copy of the software that is theirs, with nothing that renews.
What owning the copy changes
A scheduling link is a small amount of software with a large amount of data behind it. Every booking carries a name, an email address, a time and often a reason for the appointment, and the rented version of this tool keeps all of that. Running your own copy puts that list in a database on a server you chose, and lets you take it out again as a CSV or a whole-database backup whenever you like.
The commercial difference is that the meter goes away. Slotly is licensed by the number of sites you run it on -- not by calendars, not by users, not by bookings taken. Nothing renews and nothing phones home, so a copy on your server keeps working whether or not the people who wrote it are still trading. The full PHP source and the database schema come with it, which is what makes that claim checkable rather than a promise.
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What to look for
A booking page on your own domain. The link you send is the product. If it carries somebody else's brand and somebody else's domain, the relationship it builds is with them.
Per-person availability with real constraints. Per-weekday hours, split shifts, prep and cleanup buffers, blackout dates, a minimum notice window and a booking horizon. Without those, a link simply offers your whole working day and you spend the saved time declining.
A clash that cannot happen. Double-booking blocked at the database level, so two people accepting the same slot in the same second is a refusal rather than a mess.
Time zone correctness on both sides. The visitor sees their own zone, you work in yours, and impossible clock times on the spring change are never offered.
A calendar feed you can revoke. Token-guarded read-only .ics feeds per person or for the team, poll-based, so your own calendar shows the bookings without anything being granted write access.
The limits, stated plainly
The biggest behavioral difference from a rented scheduling link is the direction of the calendar connection. Slotly's feeds run one way -- out to your calendar -- and an external calendar is never read back. A rented link that reads your Google calendar and hides a slot when you accept a dentist appointment is doing something this does not do: here, availability comes from the hours and blackouts you set, not from a private diary somewhere else.
Beyond that: no cards are processed, so deposits are quoted and tracked and paid on your own payment page. One business per installation. Reminders wait on one cron line and email goes through your own SMTP server. The autumn clock change offers the repeated wall-clock hour once. And it is not a marketplace.
Questions people ask
Is there a free version? There is no free tier here. It is a one-time purchase with the source included, which is a different bargain from free: you pay once and nothing renews, and what you get is a copy rather than an account.
What happens if the people who wrote it disappear? The copy on your server keeps working. You hold the complete PHP source and the schema, nothing in it calls home, and no license check can expire against a server that has gone away.
Can I move it to a different host later? Yes. It is PHP with PDO and either MySQL, MariaDB or SQLite, so the move is a file copy and a database restore, and nothing about the license is tied to a particular machine beyond the number of sites it covers.
Where to look next
The product page is Slotly, with the feature record, the limits and what the install needs. The scheduling-link alternatives page covers the tool most people are leaving when they type this query, and the side-by-side sets the two next to each other. For what any of the rented links charges, read the observatory: every figure sits beside the date it was read and a link to the vendor page that carried it. Prices are on the observatory; read them on the date shown there.