Extend until keyword to support duration for backward scheduling in Gantt charts

[orzechow]

Proposal

Problem Statement

Gantt charts currently support the until keyword for specifying task end dates:

• task :taskId, until 2026-03-01 - works with absolute dates
• task :taskId, until milestone - works with milestone references

However, until does not support duration-based backward scheduling. You cannot specify a duration instead of a start date:

• task :taskId, 7d, until milestone or
• task :taskId, 7d, until 2026-03-01

This makes it cumbesome to define tasks that work backward from deadlines like for:

• Code sprints for fixed release dates
• Exam/Interview preparations
• Event-driven deliverables

Currently, users must manually calculate start dates and update them whenever deadlines shift.

Proposed Solution

Extend the until keyword to accept duration + reference syntax and calculate task's start date automatically.

Example

gantt
    title Release Planning
    dateFormat YYYY-MM-DD
       
    section Deadline-driven Project
    Planning                :planning, 7d, until dev
    Feature Development     :dev, 30d, until codefreeze
    Code Freeze             :codefreeze, 7d, until release
    Release Date            :milestone, release, 2026-03-01, 0d

    section Exam Preparations
    Read Book               :book, 7d, until slides
    Skim Slides             :slides, 7d, until exercise
    Exercise                :exercise, 7d, until 2026-04-01

Syntax:

<task_name> :<duration>, until <taskId>
<task_name> :<duration>, until <endDate>

Screenshots

No response

[Shahir-47]

@orzechow implemented this in #7409.

Gantt now supports duration, until <taskId>, so the start date is auto-calculated from the duration and until reference.

Preview: https://deploy-preview-7409--mermaid-js.netlify.app/

[orzechow]

Woah, awesome! That was fast 🤩
Thanks 🙏

That works great for me!

---

Not explicitly mentioned in my request, but would fit in perfectly:

Support for duration, endDate.

AFAIU your PR supports duration only with until.
What do you think?

[Shahir-47]

No problem, happy to help. Hopefully, it gets merged just as fast!

[Shahir-47]

Not explicitly mentioned in my request, but would fit in perfectly:

Support for duration, endDate.

AFAIU your PR supports duration only with until. What do you think?

Good point. Right now, the PR only supports duration, until <taskId>.
Do you want to update this issue to include duration, endDate, or would you prefer that as a separate issue?
Based on that, I can extend this PR or open a new one.

[orzechow]

Good point. Right now, the PR only supports duration, until <taskId>.
Do you want to update this issue to include duration, endDate, or would you prefer that as a separate issue?
Based on that, I can extend this PR or open a new one.

Nice! I'll add it to this issue as these are closely related.

[Shahir-47]

I will update my pr and keep you informed

[orzechow]

You could either extend the until syntax with an end date duration, until endDate or directly with the end date duration, endDate.
I guess the former would actually be more consistent with the remaining syntax 🤔

[Shahir-47]

I agree. Using duration, until <endDate> makes more sense and keeps the grammar consistent. I’ll follow that approach.

[orzechow]

Thanks! I've updated the issue

[Shahir-47]

@orzechow, I added the <duration>, until <endDate> feature. It should now support <duration>, until <endDate | taskId>.

[orzechow]

Great! Works perfectly 👌

[lee-treehouse]

Fantastic, I’ll use this a lot as I produce Gantt charts from software logs (which tend to feature completed time stamp and duration, so I generally convert them to a start time with an intermediate step).