A method and a system of maintaining slack continuity in incremental statistical timing analysis includes using a computer to forward propagating both scalar and statistical arrival times in a single timing environment; computing for a timing end point one or more projected statistical slack value; computing a scalar reverse engineered required arrival time from the projected statistical slack value; back propagating the scalar reverse engineered required arrival time using scalar delay values, measuring a resulting slack and performing a redesign based on the reverse engineered scalar required arrival time and resulting slack; and incrementally re-executing selected steps to re-compute a new scalar reverse-engineered required arrival time and new resulting slack.