Build a calendar definition

Open a calendar and select Definition. Definition changes are drafts: the page shows Unsaved changes until you choose Save or Discard. Use Cmd/Ctrl+S to save.

While Saving…, fields and draft actions are locked. If saving fails, your draft stays in place so you can correct it or try again.

Invalid or incomplete fields block saving and show errors near the affected section. Switching tabs keeps Definition and Events drafts. Leaving through the sidebar, search, project picker, page back action, or browser Back asks whether to Discard both drafts or Keep editing. If a save is in progress, navigation waits for it. A failed save keeps you on the calendar with your draft.

Set the epoch and primary chain

The epoch label describes the calendar’s origin. The primary chain decides how ticks roll up into the units and spans used to format dates.

Calendars with secondary unit chains can be summarized by the current UI but are structurally read-only. This prevents the builder from losing a definition it cannot safely edit.

Define units

Expand a unit to edit its name, structure, spans, or leap rules. Its collapsed row shows the fixed duration or the number of spans and leap rules.

Ticks are the fixed base and cannot be removed. Add larger units above it, rename them, reorder them, or delete non-base units.

A larger unit can be:

• Fixed: a factor multiplied by the previous unit.
• Spans: an ordered list of named spans, each with its own length.

For a spans unit, add, remove, rename, and reorder its spans. Their lengths are expressed in the previous unit.

Deleting a unit can also remove references from the primary chain, overlays, and leap rules. Unseen Lands shows a confirmation before applying that cascade.

Add leap rules

A leap rule selects a span and applies a positive or negative length delta when its predicate matches.

Predicates can express:

• a congruence such as year mod 4 ≡ 0;
• ALL conditions;
• ANY conditions;
• NOT around another condition.

Groups can be nested to the supported depth. Converting a populated group back to a simple congruence discards its children, so Unseen Lands asks for confirmation.

Add overlays

An overlay adds a repeating label cycle—such as weekdays—without changing unit lengths. Expand an overlay to edit it. Give it a name, choose its unit and offset, then add and reorder its cycle labels. The collapsed row shows its label count and unit.

Overlays can be used by formatted output, the jump grid, and recurring calendar events. Save the definition before expecting the Converter or Events preview to use your latest changes.