Gantt chart using relative dates?

I am helping to manage project tasks and trying to show the timing of these tasks in an Excel worksheet Gantt chart (using conditional formatting to color the cells). The trouble is that when we first put the plan together, actual dates are unknown, just dates relative to our contract being signed. Any idea how to accomodate relative dates and still be able to produce a Gantt chart from them? (usually these are indicated by ranges like Day 1 to Day 30, etc.)