During some planning and requirements gathering stages, the actual start date of releases and even beginning of development for certain products is not known. Being forced to set absolute dates for every release is awkward and leads to misunderstandings. If PM's could set a product status to "planning" and have a relative start date of "n" where all releases are set as "n+1 week", "n+8 weeks" etc. instead of using absolute date fields, then when the final go ahead is received or planning reaches a more substantial phase, the PM can simply designate "n=March 30th 2015" and all releases and other date objects will be computed from relative to absolute based on their values relative to n. Roadmap views would reflect the same n+ structure and dates would also be converted to absolute when n is defined.
Releases currently have concepts of internal and external release dates which can help with some the unknown factors when planning releases. These can be used in such a way where the Internal date is initial estimate on the release date and the external release date is more of an approximation (which can potentially be used for external stakeholders).
We feel that it is important to use absolute dates as it shows a commitment and goal for the release.
At this time, based on the current priorities and feedback from the community, we are unlikely to implement this feature.