Planner-list Relative timing

Hello everyone,

I am new to project management softwares on Ubuntu. Planner seems to  
be just what I need, except I really need to set a "relative"  
scheduling for some projects.

I mean, the project could start whenever my administration accepts it  
and I don't now precisely when it is going to be! Then I have to  
produce for them a previsional scheduling with a start time "t0" and  
weeks numbering "week #1, week #2,..." and so on, instead of fixed,  
absolute dates.

Hope I am clear :S

Is there a way to do it with planner ?

Thanks a lot!