About the due dates, I don't know, but I know there are a few people here who breed who may be able to tell you more specifically.
I do know that Chihuahuas, because of their size, often have problems with pregnancy. Not only are they small dogs, but the puppies are born with very large heads, so a C-Section is necessary in many cases.
The most obvious thing to do is take your dog to the vet. The vet will be able to tell you whether your dog is pregnant or not and give you an estimate on when the pups will be born. And most ESPECIALLY with a Chihuahua, given that you are not experienced in breeding, your vet should become your best friend for a while. If your dog needs a C-Section, the vet will have to perform it, and in a timely manner.
Gestation is, I believe, about 60 days in dogs. And your dog CAN DIE if something goes wrong. I have also heard of false pregnancies being common, which carry their own health risks. And animals can even be carrying dead pups, which can decompose in the womb.
So, VET.
Here is a site with a detailed pregnancy calendar.
http://www.ivis.org/advances/Concannon/concannon/chapter_frm.asp?LA=1
Here is a site with some information about canine pregnancy
http://allsands.com/Pets/Dogs/caninepregnancy_sdx_gn.htm