Tasks with custom dates in a workflow

It would be great if we could set up a workflow—a trigger plus creating a task with a custom date—so we can follow up on the dates our clients give us.

Right now, we only have preset dates—for example, 1 day or 2 days—and that doesn’t work when a client asks to be contacted in the middle of the month, at the end of the month, or in the first few days of June.