Cumulative Days in Gantt Chart
 

Is it possible to display cumulative days along the X axis in the Gantt chart? I need to plan a project using cumulative days, rather than calendar days.

Not sure it's exactly what you're after, but this might help.
1) Open the Project: Information inspector
2) Change the Start: to "Undetermined Date"
This will change the headings to things like "Day 17" and "Week 3"

Great suggestion
 

Your suggestion will work. I had hoped to get cumulative days, but cumulative weeks works for my project. Thanks!

You can have cumulative days by zooming in more. Click the magnifying glass in the upper right corner of the Gantt chart and choose Days.