1980

• September 19 - Hermione Granger born (why 1980 and not 1979?)

• Hermione attends Muggle primary school. 

• September 1 - Hermione meets Ron, Harry, and Neville aboard the Hogwarts Express
• September 12 - first flying lesson for Gryffindors and Slytherins
• September 19 - Hermione's 11th birthday (or 12th - her age is not certain)
• October 31 - Hermione is rescued from a Mountain Troll by Ron and Harry. The three become friends.

• first week of June - Harry, Hermione, and Ron follow Quirrell after the Sorcerer's Stone, overcome the various protections, and defeat Quirrell/Voldemort's attempt to steal the Stone 
• September 1 - Hermione and her friends starts their second year at Hogwarts
• October 31 - 500th Deathday Party for Nearly Headless Nick (CS8) 
• December 25 - Hermione takes Polyjuice Potion and accidentally transforms herself partially into a cat

• beginning of February - Hermione leaves the hospital, cured of cat whiskers and a tail
• May 8 - Hermione petrified by the Basilisk
• May 30 - Hermione cured of Petrification
• August 31 - Hermione meets Harry and Ron in Diagon Alley. They are all staying at the Leaky Cauldron.
• September 1 - Hermione and her friends begin their third year at Hogwarts, which is now surrounded by Dementors 
• Hermione is given a Time Turner by McGonagall and uses it to add hours to her day; she uses the extra time to take additional classes

• June 6 - at sundown, Buckbeak is scheduled to be executed; Sirius Black is captured that evening and narrowly escapes suffering the Dementor's Kiss by riding to freeedom on Buckbeak, through the efforts of Hermione and Harry using the Time Turner
• Monday, August 22 - Hermione attends the Final of the 422nd World Quidditch Cup
• September 1 - Hermione and her friends start their fourth year at Hogwarts
• December 25 - Yule Ball, 8 PM until midnight, Hermione's date is Viktor Krum

• June - Hermione captures Rita Skeeter in insect form

It is worth pointing out that Hogwarts doesn't necessarily follow what current British schools do, since 1) it was founded long before the current British school system was established, 2) it is really located in Scotland, which has different rules, and 3) it's part of the Wizarding World, which doesn't follow British rules on things anyway.

Here are some excellent comments from Ebony on HP4GU:

OK--first off, I realize I have less authority to speak on this, being an American.  But I dined with several Brits from the lists last weekend and we talked about it... I was the only one who is in the "Hermione was born in 1980, of course" camp.

But I think the 1979 date for Hermione's birth is incorrect. If she were nearly a year older than her classmates, I think this not only would have been more obvious in the narrative, but her characterization would have been a bit different as well IMO.

The only way I can rationalize Hermione being 12, 13, 14, and 15 years old in the respective books, in light of the way that JKR has portrayed her, is that *if* she had to sit out of school for nearly a year, she might feel as if she needs to prove something academically.

British or not, I don't *think* JKR intended for Hermione to be 
nearly a year ahead when she gave the September 19 date.  If she did, I don't think she would have put her stamp of approval on the actress who she ended up choosing for the movie.  From the glimpses I've seen on the trailer, I don't think that Emma Watson is supposed to be portraying a 12 year old.

As someone who just spent three years teaching adolescents year-round, I know firsthand that the differences between each age in the 9-14 bracket is PHENOMENAL.  And the gradations of personality change seem to vary according to gender too... respectively speaking, looking at the other characters around her, Hermione can't be a year ahead.  She's mature, but she isn't *that* mature.

We already KNOW how Jo is with numbers... we've already run into several logical contradictions in the canon (Marcus Flint repeating a grade, L&J/MWPP/Snape ages, etc.).

While I appreciate the arguments for the 1979 birthdate as an 
educator (I've spent the entire summer studying the education system in the UK and Wales), as a writer I definitely think Hermione's birthday is September 19, 1980.  It just makes more sense.  If Hermione were older, I can't help but shake the feeling that something more might have been said about it and that JKR (who is EXCELLENT at spot-on character creation) would have made it more obvious.

We have a legal birthday cutoff in my state (students by law must be five years old by December 1st of the year they begin kindergarten), and my baby sister (born February 6) had to wait until the next fall to begin school because of it.  But I graduated from high school with kids who were as young as 15 and as old as 19... many of my friends who were on the "younger" side of things had educated parents who either got their four-year olds born in the autumn/winter into kindergarten by any means necessary or, failing that, even put them into PRIVATE school so that they could begin school during the year they turned five.  Do you mean to tell me that the British system is so rigid that there is no room for moving that date?  Those of you who are currently in British schools... do you mean to tell me that all of the kids in your school fell exactly between the September 1-August 31 boundaries?  I'll e-mail a couple of the headteachers I've met this summer today to get their take on this issue.

We can pick up this debate in one and a half months' time, I 
suppose... on 9/19/01, when half of us are wishing Hermione a happy 21st and the other half are saying "no, no!  She's 22!"  And then there are those people who think we've got the dating all wrong and will be wishing her a happy 15th or a happy 16th... *goes cross-eyed*.

--Ebony AKA AngieJ - copyright 2001, used by permission
NOTE: this was written on Aug 5. By the time we all got to September 19, we all had other things to think about and never even talked about Hermione's birthday...