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.

A calendar is a CalendarData object: an id plus an optional worktime with three parts:

• hours: the working ranges of a day, as strings like "8:00-12:00". Default: ["8:00-12:00", "13:00-17:00"].
• days: seven flags, true/false or 1/0. Index 0 is Sunday. Default: Monday to Friday.
• dates: exceptions keyed by "YYYY-MM-DD". false (or 0) makes the date a day off, true (or 1) makes it a working day with the calendar's hours, and an array of ranges makes it a working day with its own 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 in calendars with the id "global" replaces it.

Which calendar a piece of work uses depends on config.calendar_mode (see SchedulingConfig):

"resource" follows MS Project. "task" and "task-inherited" suit data that only has task calendars.

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

The engine validates calendars when you pass the data to project.solve(data) or schedule. These cases throw an InputError, with path set to the offending field where the engine can name it:

• a duplicate calendar id
• a calendar with no working time
• an hour range outside 00:00-24:00
• a task or resource calendar_id that names no calendar (for resources, only in "resource" mode)
• in "resource" mode, a task calendar and a resource calendar that share no weekly working time (reported at assignments[i])

Calendars are part of the snapshot only. A change set has no calendars bucket, so you can't add or edit a calendar through apply or preview. To change one, send the full data again with project.solve(data). Edits can still point tasks and resources at existing calendars through calendar_id.

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:

import { schedule } from "plen-js";

const result = schedule(
  {
    config: { duration_unit: "hour" },
    calendars: [
      {
        id: "global",
        worktime: {
          hours: ["9:00-17:00"],
          days: [0, 1, 1, 1, 1, 1, 0], // Sun..Sat
          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 }], // Monday
  },
  { includeUnchanged: true },
);

for (const u of result.updates) console.log(`${u.id}: ${u.start_date} -> ${u.end_date}`);

Output:

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.

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

Task calendars#

Set a task's calendar_id to run that task on a different calendar. Here the migrate task runs on a night shift, while review stays on the default "global" calendar:

import { schedule } from "plen-js";

const result = schedule(
  {
    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" },
    ],
  },
  { includeUnchanged: true },
);

for (const u of result.updates) console.log(`${u.id}: ${u.start_date} -> ${u.end_date}`);

Output:

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's days are the default Monday to Friday. 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 without a link calendar 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. Switch calendar_mode to "task-inherited" to make a task without a calendar_id use the nearest ancestor's:

import { schedule } from "plen-js";

const result = schedule(
  {
    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" },
      // Friday; no calendar_id of its own.
      { id: "pour", parent: "phase", start_date: "2026-08-14 08:00", duration: 16 },
    ],
  },
  { includeUnchanged: true },
);

for (const u of result.updates) console.log(`${u.id}: ${u.start_date} -> ${u.end_date}`);

Output:

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. With the default mode it would wait for Monday.

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 a calendar through its calendar_id. The example runs the same project in "resource" and "task" mode:

import { schedule } from "plen-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: [
      // No resources: runs on "global" 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 result = schedule(project(mode), { includeUnchanged: true });
  for (const u of result.updates) {
    console.log(`${mode} ${u.id}: ${u.start_date} -> ${u.end_date}`);
  }
}

Output:

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 engine picks an assignment's calendar like this:

• The resource has a calendar and the task doesn't: the resource's calendar.
• Both have one: their intersection. Give a task a mornings 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.

Flat time#

Set config.use_calendars to false to schedule without working hours. Every calendar becomes 24/7, so a day of duration is 24 hours:

import { schedule } from "plen-js";

const result = schedule(
  {
    config: { use_calendars: false },
    // Friday; duration in days.
    tasks: [{ id: "burn-in", start_date: "2026-08-14 08:00", duration: 2 }],
  },
  { includeUnchanged: true },
);

for (const u of result.updates) console.log(`${u.id}: ${u.start_date} -> ${u.end_date}`);

Output:

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, so you can toggle use_calendars without touching the rest of the data. In flat mode resource and assignment edits never move dates.

use_calendars defaults to true. Send it explicitly when your data comes from a source with its own working-time switch.

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.

import { Project } from "plen-js";

const project = new Project();
project.solve({
  config: { duration_unit: "hour" },
  tasks: [{ id: "deploy", start_date: "2026-08-13 08:00", duration: 4 }], // Thursday
});

// The user drags the task onto Saturday 10:00.
project.apply({ tasks: { update: [{ id: "deploy", start_date: "2026-08-15 10:00" }] } });

const result = project.solve(null);
for (const u of result.updates) console.log(`${u.id}: ${u.start_date} -> ${u.end_date}`);

Output:

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 project.apply and project.preview, 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 task update.