Yes, you need a timeline when things are locked (well, more or less). Script, props, basic blocking all need to be fixed at some point so lights/sets/costumes can do their work in a timely manner. Even tho things will change in tech, even tho things will change up to opening (and, sometimes, past), you need a series of dates when you accept specific aspects of the structure, like the order of scenes, the scenes themselves, key props, which won't change again past that point (barring a massive epiphany and a cancelled performance to use the time for rehearsal by the director). And you need to get drop deads from your designer/dept heads to know when those need to be set, then back it up a few days so you have a cushion.