Abhishek Desai (right) is co-founder of Digicorp, an IT service company based in India that uses Basecamp as its primary project management tool. Desai wrote up a basic tutorial on how to do project management using Basecamp, especially in an IT or software development company. An excerpt:
Each milestone should have a date associated with it. Don't be afraid of putting dates on milestones (keeping in mind holidays and leaves of course).
As we all know nothing is certain in this world and any random event can disrupt your plan. But then you can always shift the milestones.
If you shift the first one, Basecamp gives you option to shift all the following milestones. When you do that, make sure you enter proper reason in comment. It's always better to tell the client upfront about the delay rather than on the delivery date.