Scheduling Fundamentals
Data Date
The data date is the 'as of' date a schedule update reflects -- everything before it is treated as actual progress, everything after it is forecast.
Definition
Every schedule update is really a snapshot taken as of a specific date: activities with actual dates before it are done (or in progress), and everything projected to happen after it is the schedule's forecast, driven by remaining duration and logic. The data date is also the anchor a schedule's own internal consistency gets checked against -- an actual start recorded after the data date, or a forecast start sitting before it, is logically impossible and usually means bad data entry, not a real scheduling decision. In a delay dispute, getting the data date right matters directly: a Time Impact Analysis inserts its fragnet into the update whose data date is closest to when the delay event actually happened, and analyzing the wrong update because the data date was misread means analyzing the wrong state of the project entirely.
Example
A schedule update with a March 15 data date shows 'Pour Foundation' with an actual finish of March 10 (already happened) and 'Erect Steel' forecast to start March 20 (still projected) -- both consistent with a March 15 snapshot.