The optimization is based on the the current postions of the portfolio. The date range should match for all stocks but is independent of when the securities were acquired. The aim of the optimization is to balance the weightings for the current positions based on the relationships between the securities over th past (regardless of when they were acquired).
In choosing the time range for the input, a balance is needed to to sought between recent market dymanics and length of observation period that encompasses the maximum fit of relationships that can be assumed to carry forward. Typical applications include 1 year daily or 5 year monthly depending on trading frequency within the portfolio.
