The 1970s
Events of this decade:
During the 1970s, Voldemort began to gather followers who were eager for a share of his power. The closest followers of Voldemort were known as the Death Eaters. These years got progressively more dangerous and violent. As Voldemort's influence grew, some witches and wizards were controlled by the Imperius Curse and forced to commit atrocities, including the torture and murder of Muggles. Voldemort capitalized on the prejudice and distrust between the giants and the Wizarding World; the giants became Voldemort's followers and killed and tortured many people. The Dark Mark is used by Death Eaters to indicate a house where they have committed murder. No one dared to become friendly with strange witches and wizards, not knowing if they were Voldemort supporters. Hogwarts became one of the few safe places in the Wizarding World. Bartemius Crouch is head of the Department of Magical Law Enforcement; the Aurors are authorized to use drastic methods not differing greatly from the tactics used by the Death Eaters they fight
(QA8)
c. 1970 (Y-10)
(PA18)
This is based on Lupin's birthyear.
(PA18)
"It took them the best part of three years to work out how to do it.... Finally, in our fifth year, they managed it."
(PA18)

From OP28, we know that Snape, Black, Lupin, Pettigrew, and Potter were all in the same year.
For more on the year, see [Marauders' birth year explained]
original artwork © [Jeannine Delleré], used by permission
1970 (Y-10) AD = 89 MM = 45 TR = 44 RH = 42 M = 12 BW = 0
This comes from [Dumbledore]'s statement in PS1 that "we've had precious little to celebrate for eleven years." At that time it is November, 1981. Though we don't know exactly how Voldemort's ascent to power happened, it is probably safe to assume that it somehow took a big leap in 1970.
(JKR)
1971 (Y-9) AD = 90 MM = 46 TR = 45 RH = 43 M = 13 BW = 1
[Jo Rowling] meets Ian and Vicki PotterPeter Rowling moved his family to the from Chipping Sodbury to the nearby town of Winterbourne. During the family's residence in Winterbourne, Jo and Di Rowling were friends with neighborhood children, Ian and Vikki Potter.
1972 (Y-8) AD = 91 MM = 47 TR = 46 RH = 44 M = 14 BW = 2 CW = 0
(QA7)
(JKR)
c. 1973 (Y-7)
1973 (Y-7) AD = 92 MM = 48 TR = 47 RH = 45 M = 15 BW = 3 CW = 1
(FB)
c. 1974 (Y-6)
Sirius says in OP6 that ran away from home when he was about sixteen.
(OP6)
(PA18)
1974 (Y-6) AD = 93 MM = 49 TR = 48 RH = 46 M = 16 BW = 4 CW = 2
c. 1975 (Y-5)
(OP28)
(PA18)
We don't know exactly when this happened, but it was around the beginning of the Marauders' sixth year.
(PA18)
1975 (Y-5) AD = 94 MM = 50 TR = 49 RH = 47 M = 17 BW = 5 CW = 3
c. 1976 (Y-4)
(PS9)
1976 (Y-4) AD = 95 MM = 51 TR = 50 RH = 48 M = 18 BW = 6 CW = 4 PW = 0
(JKR)
c. 1977 (Y-3)
Deduced from their birth years.
1977 (Y-3) AD = 96 MM = 52 TR = 51 RH = 49 M = 19 BW = 7 CW = 5 PW = 1
(GF16)
[Angelina Johnson] is bornOn October 31, 1994 [[Y14]], [Angelina] says her seventeenth birthday was "last week." This puts her date of birth sometime in late October, 1977 [[Y-3]].
(GF16)
c. 1978 (Y-2)
We don't know exactly when this happened, except that it was sometime between James and Lily leaving Hogwarts (c. 1977 [[Y-3]]) and Harry's birth (July 1980 [[Y0]]). Though, Molly Weasley tells us in HBP5 that couples at this time were getting married very quickly, so it's likely James and Lily were married in 1977 or 1978 [[Y-3] or [Y-2]].
For more on their ages, see [Marauders' birth year explained].
1978 (Y-2) AD = 97 MM = 53 TR = 52 RH = 50 M = 20 BW = 8 CW = 6 PW = 2 FGW = 0
(QA9)
c. 1979 (Y-1)
(PA13)
1979 (Y-1) AD = 98 MM = 54 TR = 53 RH = 51 M = 21 BW = 9 CW = 7 PW = 3 FGW = 1 HG = 0
"DON'T LIE!" bellowed Black. "YOU'D BEEN PASSING INFORMATION TO HIM FOR A YEAR BEFORE LILY AND JAMES DIED! YOU WERE HIS SPY!"
(PA19)
(BFT)
(FB)
(HBP6)
The 1980s
Events of this decade:
c. 1980 (Y0)
This interview had to have taken place sometime after July of 1979 in order for it to apply to Harry's birth in 1980 [[Y0]], and [Dumbledore] describes it as a "cold, wet night" (OP37). So it was likely made in the winter or early spring of 1979-1980 [[Y-1]-[Y0]].
We also know that Trelawney, in early September of 1995 [[Y15]], claimed she had held her post for "nearly sixteen years" (OP15), which would imply that she was hired in the middle of a school year. And then, [Dumbledore] says in June of 1996 [[Y16]] that the interview was "sixteen years ago" (OP37). So unless [Dumbledore] is wrong, the interview must have been in the first few months of 1980 [[Y0]] - sixteen years before [Dumbledore]'s statement, a slightly exaggerated 'nearly sixteen years' before Trelawney's, and during the time of year most likely to have a "cold, wet night."
1980 (Y0) AD = 99 MM = 55 TR = 54 RH = 52 M = 22 BW = 10 CW = 8 PW = 4 FGW = 2 HG = 1 H&R = 0
It is stated in [Karkaroff]'s scene in the Pensieve that he was apprehended just before [Rosier]'s death.
(GF30)
We know from Sirius Black that this happened "the year before Voldemort fell" (GF27).
We know Ernie was born in 1980 because he, Harry, and Draco Malfoy are the only students who are not yet old enough to [Apparate] on the day of the test in HBP22.
[Dudley Dursley] is bornHow do we know when [Dudley]'s birthday is? Well, we know that Harry's is July 31. A week before his birthday in 1991 [[Y11]], on July 24, Harry begins getting letters from Hogwarts. At this point, Harry is given [Dudley]'s second bedroom, where he discovers Dudley's "month-old" video camera, broken (PS3). That camera was given to Dudley for a birthday present (PS2) on Dudley's birthday, which must therefore be about a month before July 24.
No, wait, there's more. We know that in the year Harry turned 11, Harry's birthday was on a Tuesday (PS3) and [Dudley]'s was on a Saturday (PS2). Counting back a month from Tuesday, July 24, we find that the nearest Saturday is the 23rd. So regardless of the year in which this is taking place, Dudley's birthday is almost certainly on June 23.

original artwork © [Clio Chiang], used by permission
(PS2)
1981 (Y1) AD = 100 MM = 56 TR = 55 RH = 53 M = 23 BW = 11 CW = 9 PW = 5 FGW = 3 HG = 2 H&R = 1 GW = 0
In October of 1995 [[Y15]], Snape told [Umbridge] that he had been teaching at Hogwarts for 14 years, but unlike McGonagall, he did not specify the month (OP17). He later says that he took up the post on Voldemort's orders (HBP2), so it's almost certain he began teaching at the start of the school year.
Fudge says that the [Fidelius Charm] was cast "barely a week" before Voldemort came after the Potters.
(PA10)
Voldemort, nearly dead and without physical form, retreats into the forests of Albania where he possess the bodies of various animals to survive for the next decade, waiting. Hagrid, on [Dumbledore]'s orders, goes to [Godric's Hollow] to rescue baby Harry from the ruins of the house.
(PS1)
It's about 90% certain this happened on November 1, but it's also possible it was November 2.
(PA10)
but there are [twenty-four hours missing] from the chronology of events of these two days...
(PS1)
This almost certainly happened immediately following Sirius's capture on November 1, but we have no way of knowing for sure.
(GF27)
We know that this happened shortly after Voldemort's downfall, as [Dumbledore] says it was "just when everyone thought they were safe." They likely happened sometime between November 1981 [[Y1]] and very early 1982 [[Y2]].
(GF30)
after 1981 (Y1)
(PA18)
