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Kalendis

Scheduling API (keep your UI, we handle timezones/DST)

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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!

HN · launched Nov 17, 2025 · 18 upvotes (top 33%) · 1 view

Niche: appointment booking software · 120 launches, 8 in the last 7 days ▼ -60% · Explore this niche →

Category and tags

  • Horizontal
  • Dev tools
  • Developer
  • Not AI
  • Commercial product

Visit website ↗ Source listing ↗

Could you build this?

Partial Building a basic scheduling UI or CRUD booking tool is trivial, but building a battle-tested scheduling engine handling complex recurrence (RRULE), timezones, DST shifts, and race-condition-free bookings is mathematically and architecturally tricky.

What it would actually take: A robust scheduling backend requires a distributed transaction-safe database schema (PostgreSQL with strict exclusion constraints), robust RFC 5545 recurrence calculation algorithms, and IANA timezone database sync. The core difficulty lies in atomic double-booking prevention under concurrent requests across arbitrary time boundaries and Daylight Saving Time shifts. Requires backend systems expertise in distributed concurrency and temporal logic.

Discussion

8 comments analyzed.

Competitors mentioned: Acuity Scheduling, cal.com

Concerns raised: Higher API usage costs compared to cal.com, Missing features that competitors have, Unclear target audience definition

Feature requests: Additional features to close gap with cal.com

Competitors

Other products that read as similar to this one : 46 launches clear the similarity bar, closest 8 shown.

Attention rank: #22 of 47 (itself plus its competitors, highest first, normalized so YC and Product Hunt are compared fairly).

Looks like the first mover among its competitors.

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