Although it is possible to change the Time Scale in a Timeline Model so that the tick marks indicate days, weeks, months, quarters or years it is NOT possible to change what appears above the tick marks. ClarisImpact will only display dates.
Many people have requested the ability to display other types of time indicators such as:
Week 1, Week 2, Week 3, etc.
Day 1, Day 2, Day 3, etc.
Month 1, Month 2, Month 3, etc.
This is being considered for the next version of ClarisImpact.