Yes, by all means starters, alternators, etc... could be dated before the engine build. After all, they're assembled and shipped to the assembly plants in large batches. In general though, components are usually dated either shortly before, or shortly after the engine assembly date. Of course, this also depends on the application. Very common items, like low HP V8 starters or base 37 amp alternators, which are used on a lot more cars than the uncommon items, are usually dated closer to the car build date because there was more of a demand for these.
And yes, it's more important that it be dated prior to the car's assembly.
Ed