Hi Yakubu

Welcome to PP.

As follows is from the P6 Help function:

The data date is the last date you recorded progress in the form of actual dates, percent of work completed, revised remaining duration, or actual quantities or costs.

Activities are scheduled from the project data date, using the project start date as the initial data date.

Be sure to change the data date and reschedule a project each time you record progress for activities. For example, suppose you update the schedule every Friday and the current data date is Friday, November 5. When one week elapses and it is November 12, update progress through the end of the day November 11, then advance the data date to November 12. The schedule revises based on the actual dates, delays, and other changes you recorded during the past week.

Always assume that the data date starts the workperiod; the data date can be the beginning of the hour, day, week, or month. Work accomplished on the data date is not included because it is the beginning of the period.

During the project, the data date moves forward toward the project completion date. The actual dates can serve as historical data for planning another project.

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In other words: DD is the status date of your project. Everything before the DD is past; everything later or equal to DD is planned or forecasted.

Have a good planning!

Dieter