Gantt Duration column does not match with the dates column (start_date and end_date)

Hummingsoft · July 2, 2025, 3:39am

Hi,

I have been using DHTMLX gantt to show tasks from the thrid party applications like primavera P6 / Microsoft Project and once loaded these data into DHTMLX gantt, the user will be able to edit the data, I know that the gantt’s duration calculation is not inclusive, to resolve it I have added

    gantt.templates.task_end_date = (date: any): any => {
      return gantt.templates.task_date(new Date(date.valueOf() - 1));
    };

and

    const gridDateToStr = gantt.date.date_to_str(GridDateFormat, false);
    gantt.templates.grid_date_format = (date, column) => {
      if (column === "end_date") {
        return gridDateToStr(new Date(date.valueOf() - 1));
      } else {
        return gridDateToStr(date);
      }
    };

but still I am not getting the correct duration, what I intent to get the duration is inclusive of the start date and end date.

Please check the snippet here https://snippet.dhtmlx.com/kmwtxler

Valeria_Ivashkevich · July 2, 2025, 4:03pm

Hello,

Thank you for your patience while we investigated the duration calculation issue. You were on the right track with your implementation, and I’d like to clarify why the behavior occurred and how to resolve it properly.

Your initial approach is correct. However, the method used to subtract time had a problem:

  const gridDateToStr = gantt.date.date_to_str("%D %d/%m/%Y");
  gantt.templates.grid_date_format = (date, column) => {
    if (column === "end_date") {
      return gridDateToStr(new Date(date.valueOf() - 1)); // Subtracts 1 millisecond (not 1 day)
    } else {
      return gridDateToStr(date);
    }
  };

This code shifted the timestamp by 1 millisecond (not 1 day):

new Date(date.valueOf() - 1) // Subtracts 1 millisecond (not 1 day)

date.valueOf() returns a timestamp in milliseconds, from which we subtract 1 millisecond, which is why the day doesn’t roll back as we expect, and we get this: 3 April 16:00 → 3 April 15:59:59 instead of 3 April 16:00 → 2 April 16:00.

To reliably subtract 1 day, use the built-in gantt.date.add() method:

gridDateToStr(gantt.date.add(date, -1, "day")); // Subtracts 1 full day

It ensures the subtraction accounts for 24 hours (not 1 ms).
Here’s a working example: DHTMLX Snippet Tool.

Option 2:
Or you can solve this issue using duration_unit + duration formatter. This approach avoids manual date math and gives you precise control over task durations.

Configure storage unit (hours or minutes):

gantt.config.duration_unit = "hour";
gantt.config.duration_step = 1;

Initialize the Duration Formatter:

const formatter = gantt.ext.formatters.durationFormatter({
    enter: "day", 
    store: "hour", 
    format: "day",
    short: true,   
    hoursPerDay: 8,
    hoursPerWeek: 40,
  });

This plugin allows you to manage how duration is stored and display it in units you need.

Connect formatter to grid and lightbox. Since inline editing in Grid is enabled, you also need to add the formatter object to the durationEditor object via the formatter attribute:

const durationEditor = {
    type: "duration",
    map_to: "duration",
    formatter: formatter,
    min: 0,
    max: 100,
  };

  gantt.config.columns = [
    {
      name: "duration",
      label: "Duration",
      width: 100,
      align: "center",
      template: (task) => formatter.format(task.duration),
      editor: durationEditor,
      resize: true,
    },
    // ...
  ];
  gantt.config.lightbox.sections = [
    { name: "description", map_to: "text", type: "textarea", height: 70 },
    { name: "time", map_to: "auto", type: "duration", formatter },
  ];

By default, both MS Project and dhtmlxGantt assume these working hours:

8:00–12:00, 13:00–17:00 → total 8 hours per day
So:
If a user sets 8:00–16:00, that only includes 7 working hours (8:00–12:00, 13:00–16:00). To get a full 8 hours, the end time must be 17:00, not 16:00.

Please see how it can be implemented: DHTMLX Snippet Tool.

Thank you for reporting this issue. Let us know if this helps or if you need further assistance.

Best regards,
Valeria Ivashkevich
Support Engineer

Hummingsoft · July 8, 2025, 8:50am

Greetings @Valeria_Ivashkevich,

That does not resolve my issue with the duration, let me explain in bit more detail.
The data is retrieved from Primavera P6 using one of our backend system. The data from our API is correct comparing to the Primavera P6 Application.

The above screenshot is from DHTMLX Gantt implementation, where Task No: 13 is having a relationship with Task No: 15 which is a FS(Finish-Start) but if you look at the gantt it is not, one is ending and the other one is starting on the same date. By right the Task No: 15 should be starting on 14/03/2016 as Sunday is holiday, and the duration supposed to be 24 days not 23 days.

The above is the reference from Primavera P6 Application.

The end date is also coming from our backend API. Sample data can be found here https://snippet.dhtmlx.com/1zutf7oe. We have 2 use cases, use the existing from our API or create new Gantt.

The below given is my config (Copied from gantt instance debug).

{
  "links": {
    "finish_to_start": "0",
    "start_to_start": "1",
    "finish_to_finish": "2",
    "start_to_finish": "3"
  },
  "types": {
    "task": "task",
    "project": "project",
    "milestone": "milestone",
    "placeholder": "placeholder"
  },
  "auto_types": true,
  "duration_unit": "day",
  "work_time": true,
  "correct_work_time": true,
  "skip_off_time": false,
  "cascade_delete": true,
  "autosize": false,
  "autosize_min_width": 0,
  "autoscroll": true,
  "autoscroll_speed": 30,
  "deepcopy_on_parse": false,
  "show_links": true,
  "show_task_cells": true,
  "static_background": true,
  "static_background_cells": true,
  "branch_loading": true,
  "branch_loading_property": "hasChild",
  "show_loading": false,
  "show_chart": true,
  "show_grid": true,
  "min_duration": 3600000,
  "date_format": "%Y-%m-%d %H:%i:%s",
  "start_on_monday": true,
  "server_utc": false,
  "show_progress": true,
  "fit_tasks": true,
  "select_task": true,
  "scroll_on_click": true,
  "smart_rendering": true,
  "preserve_scroll": true,
  "readonly": false,
  "container_resize_timeout": 20,
  "deadlines": true,
  "date_grid": "%D %d/%m/%Y",
  "drag_links": true,
  "drag_progress": false,
  "drag_resize": true,
  "drag_project": true,
  "drag_move": true,
  "drag_mode": {
    "resize": "resize",
    "progress": "progress",
    "move": "move",
    "ignore": "ignore"
  },
  "round_dnd_dates": true,
  "link_wrapper_width": 20,
  "link_arrow_size": 12,
  "root_id": 0,
  "autofit": false,
  "scale_offset_minimal": true,
  "inherit_scale_class": false,
  "scales": [
    {
      "unit": "month",
      "step": 1,
      "format": "%F, %Y"
    },
    {
      "unit": "day",
      "step": 1,
      "format": "%D"
    }
  ],
  "time_step": 60,
  "duration_step": 1,
  "task_date": "%d %F %Y",
  "time_picker": "%H:%i",
  "task_attribute": "data-task-id",
  "link_attribute": "data-link-id",
  "layer_attribute": "data-layer",
  "drag_lightbox": true,
  "sort": false,
  "details_on_create": false,
  "details_on_dblclick": false,
  "initial_scroll": true,
  "task_scroll_offset": 100,
  "order_branch": "marker",
  "order_branch_free": false,
  "bar_height": 13,
  "bar_height_padding": 9,
  "min_column_width": 70,
  "min_grid_column_width": 70,
  "grid_resizer_column_attribute": "data-column-index",
  "keep_grid_width": false,
  "grid_resize": true,
  "grid_elastic_columns": "width",
  "show_tasks_outside_timescale": false,
  "show_unscheduled": true,
  "resize_rows": false,
  "task_grid_row_resizer_attribute": "data-row-index",
  "min_task_grid_row_height": 30,
  "row_height": 25,
  "readonly_property": "readonly",
  "editable_property": "editable",
  "calendar_property": "calendar_id",
  "resource_calendars": {},
  "dynamic_resource_calendars": false,
  "inherit_calendar": false,
  "type_renderers": {},
  "open_tree_initially": true,
  "optimize_render": true,
  "prevent_default_scroll": false,
  "show_errors": false,
  "wai_aria_attributes": true,
  "smart_scales": true,
  "rtl": false,
  "placeholder_task": true,
  "horizontal_scroll_key": "shiftKey",
  "drag_timeline": {
    "ignore": ".gantt_task_line, .gantt_task_link",
    "render": false
  },
  "drag_multiple": true,
  "csp": "auto",
  "resource_property": "resources",
  "resource_store": "resource",
  "resource_render_empty_cells": false,
  "resource_assignment_store": "resourceAssignments",
  "process_resource_assignments": true,
  "show_empty_state": false,
  "baselines": {
    "datastore": "baselines",
    "render_mode": false,
    "dataprocessor_baselines": false,
    "row_height": 16,
    "bar_height": 8
  },
  "editor_types": {
    "text": {},
    "number": {},
    "select": {},
    "date": {},
    "predecessor": {},
    "duration": {}
  },
  "touch_drag": 75,
  "touch": false,
  "touch_feedback": true,
  "touch_feedback_duration": 1,
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    },
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      },
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        "haveCustomWeeks": false,
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      }
    }
  },
  "auto_scheduling": true,
  "auto_scheduling_descendant_links": false,
  "auto_scheduling_initial": true,
  "auto_scheduling_strict": true,
  "auto_scheduling_move_projects": true,
  "project_start": null,
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  "schedule_from_end": false,
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  "tooltip_timeout": 30,
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  "tooltip_offset_x": 10,
  "tooltip_hide_timeout": 30,
  "undo": true,
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  "auto_scheduling_compatibility": true,
  "reorder_grid_columns": true,
  "risk_store": "risk",
  "risk_property": "risks",
  "scale_height": 30,
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}

I have added the default calendar and task specific calendar as well.

Please check the snippet here https://snippet.dhtmlx.com/1zutf7oe

Valeria_Ivashkevich · July 9, 2025, 6:14pm

Hello @Hummingsoft,

Thank you for your detailed explanation and for sharing the configuration and sample data. That was helpful in clarifying the issue.

Here’s what was missing in your configuration:

It’s necessary to specify the correct duration_unit (unit should be smaller than ‘days’ to calculate the total days precisely) like this:

gantt.config.duration_unit = "hour"; *// Use hours for precise calculations*
gantt.config.duration_step = 1;

This allows Gantt to account for partial days and weekends based on your calendar configuration, and ensures consistent behavior with tools like Primavera P6 (which assumes 8-hour workdays).

Proper duration formatting. You’re already using a custom duration formatter (durationFormatter), but to ensure it’s applied when rendering the duration column, update the column config like this::

gantt.config.columns = [
// Updated duration column:
{ name: "duration", label: "Duration", width: 100, align: "center", template: (task) => formatDuration.format(task.duration), editor: durationEditor, resize: true },
//...
];

The template (task) => formatDuration.format(task.duration) ensures that the duration value (stored in hours ) is dynamically converted into days (or other units) for display, using your predefined rules in durationFormatter.

What It Does:

Converts the internal hour-based duration (e.g., 192h) into a user-friendly format (e.g., 24d), matching Primavera’s 8h/day standard.
Keeps calculations precise (in hours) while displaying intuitive values (in days), thereby avoiding rounding errors.

This bridges the gap between backend data storage and frontend readability.

I’ve applied these changes to your sample snippet for testing. You can find the updated working example here: DHTMLX Snippet Tool.

Please let me know if this helps or not.

Best regards,
Valeria Ivashkevich
Support Engineer

Hummingsoft · July 10, 2025, 5:02am

Greetings @Valeria_Ivashkevich ,

Thank you for the quick response. Yes, it is working but there is an issue with the placeholder where it shows 0.13d as the duration whereas the minimum duration should be 1 day.

I am using the same snippet you provided [DHTMLX Snippet Tool](https:// DHTMLX Snippet Tool)

Regards,

Valeria_Ivashkevich · July 10, 2025, 6:38am

Greetings @Hummingsoft,

Thank you for the follow-up.

If you want to hide the placeholder task entirely, you can disable it by setting gantt.config.placeholder_task to false:

gantt.config.placeholder_task = false;

To set the default value directly for new events, use the onTaskCreated event and define here the default duration value:

gantt.attachEvent("onTaskCreated", function (task) {
task.duration = 8; // 8 hours = 1 day
return true;
});

This way, the task will not display a fractional duration, such as 0.13d, and the minimum duration will align with your working time configuration.
Please check how it can be implemented: DHTMLX Snippet Tool.

Let me know if that resolves the issue.

Best regards,
Valeria Ivashkevich
Support Engineer