Release Template: Phase beginning based on days before release date milestone

I work with on a periodic/fixed schedule of release dates. In that scenario the date that all phases of a release must begin is dependent on duration of the phase and the time BEFORE the release date. The template does not seem to support this. Instead it’s based on time AFTER the previous phase (which, I am aware, can be negative).  I would like the release template to allow me to set a phase’s end date N days before the release date milestone or the end of the next phase.