PRJ: Slack Bars Incorrect for Elapsed Duration Tasks

MORE INFORMATION

You can examine or modify Gantt bar definitions using the Format menu's Bar Style command. Free Slack and Total Slack bars can be defined that run from Finish to Free Slack, and from Finish to Total Slack. The Delay Gantt view that comes with Microsoft Project includes a Free Slack bar definition.

Steps to Reproduce Behavior

1. Create a new project. From the File menu, choose Summary Info, and change the start date to 4/11/94 8:00 AM.
2. Create tasks T1 and T2 with durations 1ed and 0ed, respectively. T1 determines the project finish date of 4/12/94 8:00 AM.
3. From the View menu, choose More Views, and apply the Delay Gantt. The Delay Gantt includes a Slack bar definition that runs from Finish to Free Slack.
4. From the Format menu, choose Layout, and clear the Round Bars To Whole Days check box. Clearing this check box makes it easier to see how Project determines the end of the Slack bar for T2.
5. From the View menu, choose Tables, and apply the Schedule table. The last two fields are Free Slack and Total Slack. Tasks T1 and T2 have Free Slack of 0d and 1ed respectively.

The Slack bar for T2 should have ended at 4/12/94 8:00 AM, the project finish date, but instead is drawn to 4/14/94 5:00 PM. The 1ed Free Slack for T2 is stored internally as 24*60=1440 elapsed minutes. When the Slack bar is drawn, the 1440 is misinterpreted as working minutes and the end of the bar is determined using working time from the project calendar. There are 8*60=480 working minutes per working day in the default Standard calendar, so 1440 working minutes is equivalent to 1440/480=3 working days. The Slack bar for T2 is drawn with a length of 3 working days instead of 1 elapsed day.

NOTE: In the above example, the key factor is that T2 had a duration entered in elapsed units, not that its duration was 0. The 0 duration was used to make it easier to study the Slack bar.