Show HN: Kalendis – Scheduling API (keep your UI, we handle timezones/DST)

Kalendis is an API-first scheduling backend. You keep your UI; we handle the gnarly parts (recurrence, time zones, DST, conflict-safe bookings).

What it does: • MCP tool: generates typed clients and API route handlers (Next.js/Express/Fastify/Nest) so you can scaffold calls straight from your IDE/agent tooling. • Availability engine: recurring rules + one-off exceptions/blackouts, returned in a clean, queryable shape. • Bookings: conflict-safe endpoints for creating/updating/canceling slots.

Why we built it: We kept rebuilding the same "hard parts" of scheduling: time zones/DST edge cases, recurring availability, conflict-aware booking, etc. We wanted a boring, reliable backend so we could ship product features without adopting a hosted scheduling UI.

How it's helped: We stopped re-implementing DST/recurrence math and shipped booking flows faster. One small team (just 2 developers) built a robust booking platform for their business using Kalendis—they kept full control of their UX without spending lots of cycles on scheduling infrastructure. The MCP generator cut the glue code: drop in a typed client or route, call the API, move on.

Some tech details: • REST API with ISO-8601 timestamps and IANA time zones • Recurring availability + one-off exceptions (designed to compose cleanly) • Focused scope: users, availability, exceptions, bookings (not a monolithic suite)

The MCP server exposes tools like generate-frontend-client, generate-backend-client, generate-api-routes, and list-endpoints. Add to your MCP settings:

  {
   "mcpServers": {
    "kalendis": {
     "command": "npx",
     "args": ["-y", "@kalendis/mcp"]
    }
   }
  }

How to try it: Create a free account → get an API key. (https://kalendis.dev). Then hit an endpoint:

  curl -H "x-api-key: $KALENDIS_API_KEY" \
 "https://api.kalendis.dev/v1/availability/getAvailability?userId=<user-id>&start=2025-10-07T00:00:00Z&end=2025-10-14T00:00:00Z&includeExceptions=true"

Happy to answer questions and post example snippets in the thread. Thanks for taking a look!