I couldn't find the original post but wanted to add a couple things about hatch time and how it relates to incubator temp and humidity.
Just to be clear, there are exceptions to every rule. Some people run their incubators with no water (humidity) whatsoever and have good hatch rates. The only thing I can guess on this is that they are running them in an area (basement?) that has a fairly good relative humidity already and that air is being circulated in their incubators.
The main contributor to hatch timing is temperature. Over the years we've pretty much nailed down 21 days as the average time it takes a chicken egg to hatch. However, there can be variances. Man made variances come with artificial incubation. The preferred temp for a circulated air incubator is in the neighborhood of 99.5 degrees. It's very interesting how much influence a variance of a degree one way or the other can have. Variances more than that can have detrimental effects on your hatch rates. It stands to reason that a lower temp would slow down your hatch and delay it by a day or so, and conversely, higher temps would speed up your hatch by a day or so. I use two GQF Sportmans Incubators and one GQF Sportsman hatcher. They work great for me and I am very happy with my hatch rates. These days it's possible to get your incubators with electronic thermostats. My hatcher has one that is controlled by changing dip-switch settings, and you can get to about 1/8 of a degree variance depending upon the switch settings. I have my set at right around 99 degrees. I've read that during incubation the hatching chicks actually create more heat and it's recommended that you use a bit lower temperature setting in your hatcher. If you don't have a hatcher it's really not that big a deal. My incubators are both 1202's which are a little bit older models. this spring I upgraded the 2nd one (which I got from my Dad) and added an electronic thermostat (multi-turn control). Now both are outfitted with electronic thermostats. I use the thermometers that come with them, but I also verify their temps with a digital thermometer that has a probe I can just shut in the door and it hangs there. Both run at 99.5 to 99.8 degrees. I try to keep my incubators at around 50-55% RH. I use a wich pad in my hatcher which increases the surface area of evaporating water, which raises the humidity. The pan can accomodate two wick pads but I've found that one works the best (for me). I incubate in my basement and as the outside RH increases as it warms up for spring, so does the RH of my basement, so keep that in mind when adjusting vents on your incubators.
During the process of incubation the egg loses a percentage of its moisture and the air cell size increases as hatch time nears. There are charts available (just google the topic) that show how your air cell size should look through the incubation period. This is also a reliable method to guage whether your humidity is correct. There are all kinds of little tweaks that can be done in regards to humidity and you have to determine what works best for you.
As for temps. A day early or a day late assuming your temp is low or high shouldn't have much of an impact on your hatch rate. however, longer or shorter periods can effect your chicks. Chicks that labor through hatching and take longer, either due to low temps or maybe low humidity, have a higher probability of having leg/toe/foot issues. Many times crooked toes are directly related to hatching and not genetic. Look at the chicks when they are first born. Even on a new born chick at 21 days, their toes are initially curved in toward the body, sort of tucked in like a little ball so they won't obstruct the chick as it rotates inside the egg to chisel its way out of the shell. The longer that chick has to stay inside that oval compartment, the greater the tendency for those toes to want to stay in that position. Shorter hatch times could also lead to some deformities. You don't want them to develop too fast and too high temps will just kill the embryo outright, essentially cooking it.
I have noted some other things that impact hatch timing.
Hybrid vigor - take two unrelated lines or breeds. Cross them. I am betting those eggs will be the first to hatch every time.
Slow developing breeds = Late Hatching chicks: This may just be me, but my large Whites are very slow to feather, and develop into adults. That's fine with me because the end result is worth the wait, but the eggs tend to hatch last of all the breeds/varieties I raise, sometimes as many as two days late, with no toe defects.
The breed: My wyandotte and rock bantams are always first, usually at 20.5 days, the Cochins at 21 to 21.5 days, Minorcas and Leghorns 20 to 21 days.
Just some things I've noted...
BTW...my chick total for the year is at approximately 700 and counting...
