Adding 86,400,000 milliseconds a day is right until the day it isn't

Expanding a recurrence rule by adding a fixed number of milliseconds per day is correct until the zone's next DST transition, then wrong by the transition size forever after -- 30 minutes on Lord Howe, not always 60.

This is one of the things Scheduling & Availability Core already handles. Bookings and availability that survive timezones, repeating events and daylight saving — without double-booking.

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Is this you?

Computing each occurrence of 'every Tuesday at 9am' by adding a fixed 86,400,000-millisecond day to a UTC instant works correctly for every week except the ones that cross a DST transition, where the real day is 23 or 25 hours -- 23.5 or 24.5 on Lord Howe Island, which shifts by only 30 minutes, not the whole hour a naive fix would assume. After the transition, every subsequent occurrence is off by the transition size, permanently.

Why this one is easy to get wrong

Adding a fixed millisecond interval per day is the obvious, simplest way to expand a daily or weekly recurrence, and it passes any test run outside the narrow week a transition actually happens -- the drift is invisible in normal development and only appears twice a year, in whichever specific week the local DST calendar changes, which most test data doesn't happen to cover.

What you get instead

expandLocal() operates entirely on local calendar fields with no Zone parameter at all and no access to the zone-conversion module -- it expands the recurrence over LocalDateTime values first, and only maps each one to an instant afterward via instantFromLocal(). The harness asserts this directly: the same rule and dtstart, expanded for two resources differing only in timezone, produce identical local starts and different instants, and a daily rule crossing a transition produces the correct 23/24/25-hour gaps, never a lost or duplicated occurrence.

Source: ARCHITECTURE.md 'Expansion is FLOATING' — checkable in the pack you receive

How you actually use this

You don’t install a library or wire up an SDK. Your own coding agent builds the code in your project, and you keep it — no runtime dependency on us.

Step 1
Download and unzip
You get a folder: the docs that tell an agent what to build, a starting skeleton, and the test suite that decides when it's done.
Step 2
Open it in Claude Code or Cursor
Point your coding agent at the folder. Nothing to install, no account with us, no API key.
Step 3
Paste one prompt
The pack contains the exact prompt. Paste it as your first message and leave it alone — it works through the build itself, choosing a cheaper or stronger model per task.
Step 4
Run ./verify.sh
One command. It prints a pass or fail for every check. Green means the build is done — the same script we ran to produce the receipt on this page.

Typical build: about 22 minutes of your agent working, mostly unattended. Then you integrate the working module into your app the way you would any code you’d written yourself.

Why you can believe this

3 of 3 runs passed

We ran this pack from an empty folder 3 times and published exactly what happened — every check, the model, the token cost, the wall time. Not a testimonial, and not our opinion: the same verify.sh you run yourself. Read the full receipt →

Buy for $99 14-day refund if verify.sh fails →