The workflow feature uses the base date for triggering items in your workflow. By default the base date is set to the project date (which is the most common scenario).
If you choose, you can set the base date to something different. We find this most common when using multiple workflows on a single project.
For example, a photographer may have a client who is having engagement photos and then wedding photos taken. The engagement shoot date would be different than the project date. In this circumstance, you would want to change the base date to something other than the project date.
KeywordSearch: Workflow base date