gantt charts in confluence drawio: "happens" uses wrong date

[Wyphorn]

Describe the bug
When using gantt diagrams within Confluence Cloud with drawio, the syntax is not interpreted correctly for the "happen" keyword.
Check out plantuml docu how the date should be handled: https://plantuml.com/gantt-diagram#outa0ef7ea1711c2a6a1d55c31efc0e36bb
Defining the date shall place the milestone there. On drawio integration, it is normally placed one day to late:

• [fix] happens 2026-10-03 will show the "fix"-milestone on the 4th.

To Reproduce
Steps to reproduce the behavior:

1. Insert drawio macro
2. Click Arrange > Insert > Advanced > PlantUML...
3. use this short example:

@startgantt
saturday are closed
sunday are closed

Project starts 2026-10-01
2026-10-13 is closed

[fix]     happens 2026-10-03 and is colored in SlateGray
[frame]   requires 10 days
[frame]   starts at [fix]'s end and is colored in LightGray

@endgantt

4. The fix-milestone is on the 4th:

Expected behavior
The milestone should be on the 3rd.

draw.io version (In the Help->About menu of the draw.io editor):

• draw.io version 31.4.5

Desktop (please complete the following information):

• OS: current Windows 11
• Browser: current Chrome
• Browser Version: 153

I tested the problem in incognito/private mode with all browser extensions switched off, write "yes" below:
yes

Additional context

[draw-io-triage-bot]

👋 Thank you for opening this issue!

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Summary: The PlantUML Gantt chart happens keyword for milestones places the milestone one day later than the specified date when rendered in draw.io (Confluence Cloud integration), suggesting an off-by-one date parsing or rendering bug.

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📝 Additional Information Needed

To help us investigate this more effectively, could you please provide:

• Can you confirm whether this off-by-one error also occurs in the standalone draw.io desktop or web application (app.diagrams.net), or only within the Confluence Cloud integration?
• Does the issue persist if you remove the 'saturday are closed' and 'sunday are closed' lines (i.e., is it related to closed-day handling shifting the milestone date)?

📋 Bug Report Template

**Environment:**
- draw.io version: [e.g., web app / desktop 24.x.x / Confluence plugin]
- Browser (if web): [e.g., Chrome 120, Firefox 121]
- OS: [e.g., Windows 11, macOS 14, Ubuntu 22.04]

**Steps to Reproduce:**
1. Go to ...
2. Click on ...
3. See error

**Expected Behavior:**
What you expected to happen.

**Actual Behavior:**
What actually happened.

**Screenshots/Files:**
If applicable, add screenshots or attach the .drawio file.

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_{🤖 This is an automated response to help with initial triage. A human maintainer will review this issue. If you've provided all the requested information, no further action is needed - we'll get back to you soon!}

[Wyphorn]

Good point!

📝 Additional Information Needed

To help us investigate this more effectively, could you please provide:

• Can you confirm whether this off-by-one error also occurs in the standalone draw.io desktop or web application (app.diagrams.net), or only within the Confluence Cloud integration?

Same behaviour on https://app.diagrams.net/.

• Does the issue persist if you remove the 'saturday are closed' and 'sunday are closed' lines (i.e., is it related to closed-day handling shifting the milestone date)?

Interesting. removing the weekend days results in off-by-one in the other direction. It will set the "happens 2026-10-08" milestone to the 7th.
So the problem still occures, but on the other side. Also in the web app. But one way or another: the documentation of "happen" is different from what is here.