Starting from version 1.5 the widget supports showing the actual vs initial schedule. 

One time comparision

You can compare the current schedule with past dates easily by selecting the historical moment:

Show difference all time  

Since version: Gantt 2.4.0

You may want to show the difference any time you open a gantt view. The Baselines Widget properties section let you select the historical moment in the past that should be used as default reference point for comparision, so any time you open the gantt, the difference to this moment will be shown.

So open Widget properties > Baselines and turn :"Show Baselines" to YES.

You can compare it with:

  • relative date (T-minus)
  • any particular date in a history
  • or let the user pick the date using page parameters

API Approach

Since version: Gantt 1.5.0

You can also pass values using an API.

To do so, you need to use the Advanced > Item Script parameter, and fill in the value from a custom field, or load it from history.  

Let's suppose you have two additional custom fields: gantt_initial_start and gantt_initial_duration, and you have workflow action, that copies gantt_start to gantt_initial_start (resp. gantt_duration to gantt_initial_duration) during a workflow transition "approveSchedule", that you can use Advanced > Item Script  parameter 

task.planned_start_date=util.getDate(wi, "gantt_initial_start");

task.planned_duration=util.getDuration(origWi, "gantt_duration")

This will compare the schedule not a single moment in history, but it will be item-specific values.