[Canonical page](<https://docs.dhtmlx.com/planning-engine/wasm/scheduling/working-time-calendars/>)
[Full documentation index](<https://docs.dhtmlx.com/planning-engine/llms.txt>)


# Working time and calendars

This guide shows how to tell Planning Engine when work can happen: working hours, weekdays, holidays and short days. It also covers how tasks and resources pick a calendar, and how to turn calendars off.

## How calendars work

A task's `duration` is **working time**. A 16-hour task doesn't end 16 hours after it starts. It ends once it has used 16 hours of its calendar's working time, so nights, weekends and holidays stretch it. Lags work the same way. `leveling_delay` and `pauses` are the exception: they're elapsed wall-clock time.

Calendars go in [`ProjectData.calendars`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/project-data.md>). Each one is a [`CalendarData`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/calendar-data.md>) object: an `id` plus an optional [`worktime`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/calendar-data.md#worktime-fields>) with three fields:

- `hours`: the working ranges of a day, as `"H:MM-H:MM"` strings. Default: `["8:00-12:00", "13:00-17:00"]`. `"24:00"` is the only hour-24 value. A range that ends before it starts crosses midnight.
- `days`: exactly seven entries, booleans or numbers, **index 0 is Sunday**. Default: Monday to Friday.
- `dates`: exceptions keyed by `"YYYY-MM-DD"`. A value of `false` (or `0`) makes the date a day off. `true` (or a non-zero number) makes it a working day with the calendar's `hours`. An array of ranges such as `["9:00-13:00"]` makes it a working day with those hours.

Every project has a calendar named `"global"`. If you don't define one, the engine uses the built-in Monday-Friday, 08:00-12:00 and 13:00-17:00. A calendar with the id `"global"` in `calendars` replaces it.

Which calendar a piece of work uses depends on [`config.calendar_mode`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/common.md#calendar-mode>) (see [`SchedulingConfig`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/scheduling-config.md>)):

| Mode | A task works on |
|---|---|
| `"resource"` (default) | each assignment's calendar: the resource's calendar, intersected with the task's own calendar when both are set |
| `"task"` | the task's `calendar_id`, else `"global"`; resource calendars are ignored |
| `"task-inherited"` | the task's `calendar_id`, else the nearest ancestor's, else `"global"` |

`"resource"` follows MS Project. `"task"` and `"task-inherited"` use task calendars only.

`config.use_calendars` sits above all of this. It's `true` by default, and `null` also reads as `true`. Set it to `false` and every calendar becomes flat 24/7 time.

The engine checks calendars when you pass data to [`schedule`](<https://docs.dhtmlx.com/planning-engine/wasm/api/schedule.md>) or [`solve`](<https://docs.dhtmlx.com/planning-engine/wasm/api/project-solve.md>). These cases return a `validation` error envelope with the field path:

| Problem | `path` |
|---|---|
| A duplicate calendar id | `calendars[i]` |
| A calendar with no working time | `calendars[i]` |
| A task `calendar_id` that names no calendar | `tasks[i].calendar_id` |
| A resource `calendar_id` that names no calendar (only in `"resource"` mode) | `resources[i].calendar_id` |
| In `"resource"` mode, a task calendar and a resource calendar that share no weekly working time | `assignments[i]` |

The JSON decoder rejects a few shapes before the engine sees them. These come back as `validation` with `path: null`: an hour range it can't read (`"8:00-25:00"`), a `days` list without exactly 7 entries, and `customWeeks`, which isn't supported. For example:

```json
{
  "error": { "code": null, "kind": "validation", "message": "invalid hour range", "path": null },
  "ok": false
}
```

:::note
A malformed hour range fails the whole call with a `validation` error that has no `path`. In [`apply`](<https://docs.dhtmlx.com/planning-engine/wasm/api/project-apply.md>), this means no operation from that change set is applied.
:::

Calendars belong to the snapshot only. [`SchedulingChanges`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/scheduling-changes.md>) has no `calendars` field, so `apply` can't add or edit a calendar. Sending one is a `parse` error. To change a calendar, send the full data again with `solve(dataJson, ...)`. Edits can still point tasks and resources at existing calendars through `calendar_id`. An unknown id there is a rejected operation in the [`ApplyReport`](<https://docs.dhtmlx.com/planning-engine/wasm/api/project-apply.md#applyreport>), not an error envelope. See [Error handling](<https://docs.dhtmlx.com/planning-engine/wasm/error-handling.md>) for both levels.

## Defining calendars

Replace `"global"` to change the working week for the whole project. The calendar below works Monday to Friday, 09:00-17:00. Thursday the 13th is a short day and Friday the 14th is a holiday:

```js
const { schedule } = require("./pkg-node/plen_wasm.js");

const data = {
  config: { duration_unit: "hour" },
  calendars: [
    {
      id: "global",
      worktime: {
        hours: ["9:00-17:00"],
        days: [0, 1, 1, 1, 1, 1, 0], // Sunday..Saturday
        dates: {
          "2026-08-13": ["9:00-13:00"], // Thursday: a short day
          "2026-08-14": false, // Friday: a public holiday
        },
      },
    },
  ],
  tasks: [{ id: "build", start_date: "2026-08-10 09:00", duration: 36 }],
};

const response = JSON.parse(schedule(JSON.stringify(data), '{"includeUnchanged":true}'));
if (!response.ok) {
  throw new Error(`${response.error.kind}: ${response.error.message} (at ${response.error.path})`);
}
for (const update of response.result.updates) {
  console.log(`${update.id}: ${update.start_date} -> ${update.end_date}`);
}
```

Output:

```text
build: 2026-08-10 09:00 -> 2026-08-17 17:00
```

The 36 hours are Monday to Wednesday (24 h), the short Thursday (4 h) and the next Monday (8 h). The holiday and the weekend hold no work. `includeUnchanged` makes `updates` list the task even though its start didn't move.

Each `worktime` field you leave out keeps its default. `{ "days": [0, 1, 1, 1, 1, 1, 1] }` changes the weekdays and keeps the 08:00-12:00, 13:00-17:00 hours.

## Task calendars

Set a task's [`calendar_id`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/task-data.md>) to run that task on a different calendar. Here `migrate` runs on a night shift, while `review` stays on the default `"global"` calendar:

```js
const { schedule } = require("./pkg-node/plen_wasm.js");

const data = {
  config: { duration_unit: "hour" },
  calendars: [
    // 22:00-6:00 ends before it starts, so it crosses midnight
    // and belongs to the day it starts on.
    { id: "night", worktime: { hours: ["22:00-6:00"] } },
  ],
  tasks: [
    { id: "review", start_date: "2026-08-10 08:00", duration: 16 },
    { id: "migrate", start_date: "2026-08-10 22:00", duration: 16, calendar_id: "night" },
  ],
};

const response = JSON.parse(schedule(JSON.stringify(data), '{"includeUnchanged":true}'));
if (!response.ok) {
  throw new Error(`${response.error.kind}: ${response.error.message} (at ${response.error.path})`);
}
for (const update of response.result.updates) {
  console.log(`${update.id}: ${update.start_date} -> ${update.end_date}`);
}
```

Output:

```text
review: 2026-08-10 08:00 -> 2026-08-11 17:00
migrate: 2026-08-10 22:00 -> 2026-08-12 06:00
```

The night shift keeps the default Monday-to-Friday days. Monday's shift runs until Tuesday 06:00, because a range that crosses midnight belongs to the day it starts on.

A task's own calendar (else `"global"`) also matters in `"resource"` mode. Milestones, lag and slack always use it.

### Inheriting a calendar from a summary

In `"task"` and `"resource"` modes a child doesn't look at its parent's calendar. Set `calendar_mode` to `"task-inherited"` to make a task without a `calendar_id` use the nearest ancestor's:

```js
const { schedule } = require("./pkg-node/plen_wasm.js");

const data = {
  config: { duration_unit: "hour", calendar_mode: "task-inherited" },
  calendars: [{ id: "six-day", worktime: { days: [0, 1, 1, 1, 1, 1, 1] } }],
  tasks: [
    { id: "phase", type: "summary", calendar_id: "six-day" },
    { id: "pour", parent: "phase", start_date: "2026-08-14 08:00", duration: 16 }, // Friday
  ],
};

const response = JSON.parse(schedule(JSON.stringify(data), '{"includeUnchanged":true}'));
if (!response.ok) {
  throw new Error(`${response.error.kind}: ${response.error.message} (at ${response.error.path})`);
}
for (const update of response.result.updates) {
  console.log(`${update.id}: ${update.start_date} -> ${update.end_date}`);
}
```

Output:

```text
pour: 2026-08-14 08:00 -> 2026-08-15 17:00
phase: 2026-08-14 08:00 -> 2026-08-15 17:00
```

`pour` works Friday and Saturday on the summary's six-day week. Drop `calendar_mode` and it runs on `"global"` instead: Friday, then Monday, ending `2026-08-17 17:00`.

## Resource calendars

In the default `"resource"` mode each assignment works on its resource's calendar, and the task's dates follow its assignments. Give a [resource](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/common.md#resourcedata>) a calendar through its `calendar_id`. The example runs the same project in `"resource"` and `"task"` mode:

```js
const { schedule } = require("./pkg-node/plen_wasm.js");

function project(mode) {
  return {
    config: { duration_unit: "hour", calendar_mode: mode },
    calendars: [{ id: "tue-sat", worktime: { days: [0, 0, 1, 1, 1, 1, 1] } }],
    tasks: [
      // `prepare` has no resources, so it runs on the global calendar
      // and ends Friday 17:00.
      { id: "prepare", start_date: "2026-08-14 08:00", duration: 8 },
      { id: "inspect", start_date: "2026-08-14 08:00", duration: 16 },
    ],
    links: [{ id: "l1", source: "prepare", target: "inspect", type: "fs" }],
    resources: [{ id: "dana", calendar_id: "tue-sat" }],
    assignments: [{ id: "inspect:dana", task: "inspect", resource: "dana", value: 1 }],
  };
}

for (const mode of ["resource", "task"]) {
  const response = JSON.parse(schedule(JSON.stringify(project(mode)), '{"includeUnchanged":true}'));
  if (!response.ok) {
    throw new Error(`${response.error.kind}: ${response.error.message} (at ${response.error.path})`);
  }
  for (const update of response.result.updates) {
    console.log(`${mode} ${update.id}: ${update.start_date} -> ${update.end_date}`);
  }
}
```

Output:

```text
resource prepare: 2026-08-14 08:00 -> 2026-08-14 17:00
resource inspect: 2026-08-15 08:00 -> 2026-08-18 17:00
task prepare: 2026-08-14 08:00 -> 2026-08-14 17:00
task inspect: 2026-08-17 08:00 -> 2026-08-18 17:00
```

In `"resource"` mode Dana works Saturday, so `inspect` starts right after `prepare` and runs Saturday and Tuesday. In `"task"` mode her calendar is ignored and `inspect` waits for Monday on `"global"`.

The rules for picking an assignment's calendar:

- The resource has a calendar and the task doesn't: the resource's calendar.
- Both have one: their intersection. Give a task a mornings-only calendar and assign it to Dana, and the work runs Tuesday to Friday mornings only.
- The resource has none: the task's calendar.
- Summaries and milestones always use the task's calendar.

A task with several assignments on different calendars spans the **envelope** of its assignments. Each assignment starts at the first working time of its own calendar and runs the full duration there. The task ends when the last one ends. The reported `duration` stays the working time of each assignment, not the length of the envelope. Results don't include per-assignment dates.

Resource calendars also shape resource load: each assignment books work only on its own calendar. See [Resource load](<https://docs.dhtmlx.com/planning-engine/wasm/resources-and-assignment/resource-load.md>).

## Flat time

Set [`config.use_calendars`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/scheduling-config.md>) to `false` to schedule without working hours. Every calendar becomes 24/7, so a day of duration is 24 hours:

```js
const { schedule } = require("./pkg-node/plen_wasm.js");

const data = {
  config: { use_calendars: false },
  tasks: [{ id: "burn-in", start_date: "2026-08-14 08:00", duration: 2 }], // Friday, 2 days
};

const response = JSON.parse(schedule(JSON.stringify(data), '{"includeUnchanged":true}'));
if (!response.ok) {
  throw new Error(`${response.error.kind}: ${response.error.message} (at ${response.error.path})`);
}
for (const update of response.result.updates) {
  console.log(`${update.id}: ${update.start_date} -> ${update.end_date}`);
}
```

Output:

```text
burn-in: 2026-08-14 08:00 -> 2026-08-16 08:00
```

With calendars on, the same task skips the weekend and ends on Tuesday at 08:00. The engine still validates calendar ids, but they all resolve to the same flat time. That lets you toggle `use_calendars` without touching the rest of the data. In flat mode, resource and assignment edits never move dates.

When `use_calendars` is left out, the engine schedules on calendars, so send `false` explicitly for flat time.

## Explicit starts on non-working time

When an edit sets a start that falls on non-working time, the engine doesn't always snap it. A task **with no predecessor** keeps the exact start you gave it. Its end is computed forward on the working calendar, so the duration is still right. A task with a predecessor gets its start from the link instead.

```js
const { JsonProject } = require("./pkg-node/plen_wasm.js");

function unwrap(text) {
  const response = JSON.parse(text);
  if (!response.ok) {
    const { kind, code, message, path } = response.error;
    throw new Error(`${kind} ${code ?? ""}: ${message} (at ${path})`);
  }
  return response.result;
}

const data = {
  config: { duration_unit: "hour" },
  tasks: [{ id: "deploy", start_date: "2026-08-13 08:00", duration: 4 }], // Thursday
};

const project = new JsonProject("{}");
try {
  unwrap(project.solve(JSON.stringify(data), "{}"));

  // The user drags the task onto Saturday 10:00.
  const changes = { tasks: { update: [{ id: "deploy", start_date: "2026-08-15 10:00" }] } };
  const report = unwrap(project.apply(JSON.stringify(changes)));
  if (report.needsResync) {
    throw new Error("the edit was rejected");
  }

  const result = unwrap(project.solve(undefined, "{}"));
  for (const update of result.updates) {
    console.log(`${update.id}: ${update.start_date} -> ${update.end_date}`);
  }
} finally {
  project.free();
}
```

Output:

```text
deploy: 2026-08-15 10:00 -> 2026-08-17 12:00
```

The start stays on Saturday 10:00. The four hours of work run Monday 08:00-12:00. This applies to [`apply`](<https://docs.dhtmlx.com/planning-engine/wasm/api/project-apply.md>) and [`preview`](<https://docs.dhtmlx.com/planning-engine/wasm/api/project-preview.md>), the calls behind drags in an editor.

This is by design: the UI layer owns snapping. If your editor should only allow working-time starts, snap the date before you send the [`TaskUpdate`](<https://docs.dhtmlx.com/planning-engine/wasm/api/types/task-update.md>).