Ok, if I understand what you're saying correctly...
It makes no difference at all when the eggs were LAID, because they won't start to develop until the hen "starts to set" on them. By which I mean that at a certain point she will stop laying, tuck the eggs up under her, and then stay there for three weeks, turning the eggs constantly, and only getting up once or maybe twice in a twenty-four hour period. If she is still wandering in and out throughout the day she is not actually "setting." She starts incubating the eggs all at once, and quite suddenly, so that they will all hatch at once (actually, over a two-day period, to be exact).
Once she has actually started setting on the eggs she will hatch--or once she has started setting and you have placed your chosen eggs under her--you don't want to add ANY new eggs to the nest! She will still try to hatch these because she can't count, but they won't hatch when the others do and most likely will be abandoned after the first chicks hatch. Your hen may not be happy about it, but she will HAVE to abandon them or die of thirst and starve to death along with her other chicks while waiting for days and days for all the eggs to hatch, because a hen WILL NOT LEAVE the nest (even to poop) during the time the eggs are hatching! A couple days after the first eggs hatch, she will likely (and rightly) assume that something is wrong with the remaining eggs, and decide to cut her losses and leave the nest with the chicks to look for food and water.
In the "wild" or in a "free-range" setting, a hen will avoid this confusion and complication by hiding her nest where none of the other hens (or anyone else) can find it, and hatching her clutch in peace. But hens in the "unnatural" environment of some level of confinement, accustomed to laying in communal nests, need OUR HELP as responsible owners in an situation where their instincts are at odds with their surroundings. To think we can just "stop collecting the eggs and let them hatch some babies" as so many seem inclined to do is a cop-out that borders on negligence.
I want to emphasize that setting is not a passive process of the hen "just sitting on the eggs" like some machine or dumb animal; it involves a lot of active work and self-sacrifice on the part of the hen, in distinct phases starting with the initial broodiness and culminating with raising the hatched chicks. If you are unfamiliar with this process I highly recommend to do some research so you understand what's going on. You will be able to help your hen to hatch a clutch much more effectively that way and save yourself and her much grief.
This article is also a great resource for hatching with hens--I highly recommend it, as it helped me a lot:
http://www.themodernhomestead.us/article/Broody-Hens-1.html