Is there a typographic or historical explanation for why multi-digit numbers get a comma (or space or point) inserted to separate groups of three digits, but not years? If separating number groups wasn’t in practice at the time (the year 1000), it clearly wasn’t in practice for numbers in general. When it did come into being (when?), why didn’t it apply to the year? Was it theological? Was it because the year is not an adjective?
Four digit numbers usually don’t get commas in the first place. Most style guides tell you leave them out until the numbers get to five digits.
June 13, 2,008. Looks pretty funky. Is it the 13th, or 13.2?
06/13/2,008. 13/06/2,008 for you Euros. Still pretty pointless.
As elegant as this answer is, it only postpones the need for a definite answer by approximately 8000 (or 8,000?) years. Shouldn’t we on the SDMB be accustomed to long term thinking? 
Long term thinking? We never decided what we are calling this decade!
Sure we have! In just a couple more years, we’ll be calling it “over.”
The Naughts. As in Jethro Bodine and his “double naught spy” routine. Fitting when our President during most of the decade bears a lot of similarities to Jethro.
In any science fiction that I’ve ever read, the year would also be written without a comma when consisting of five digits as well. Can’t recall what was done with more digits, or recall whether I’ve seen them.
Of course a quick look at the Wikipedia article on Foundation shows commas in all the five digit years. :smack:
It could be different. We could be using Indian numeric separators:
10
100
1000
10,000
1,00,000 (100 thousand; 1 lakh)
10,00,000 (10 lakh)
1,00,00,000 (100 lakh; 1 crore)
The uh-ohs.
Cite? I’ve never seen a style guide that says this.
The Chicago Manual of Style says to use commas “In most figures of one thousand or more.” It also says to use commas in “year numbers of five or more digits.”
Wouldn’t it be cool if we all woke up after going to bed on Dec. 31, 2009, and woke up on Jan. 1, 2000, with the whole thing free to be done over again?
The thread title should read
Why don’t we put a comma in the year? (e.g., 2008, not 2,008)
I note with interest the comma following e.g.
It’s a practice I had hitherto failed to notice and I discover that you are correct, whereas a Briton, e.g. me, is permitted to omit it.
Good to know! I assume the same is true for “i.e.”?
It is indeed.
Perhaps, but this doesn’t seem correct:
Why don’t we put a comma in the year (for example, 2008, not 2,008)?
Rather, I meant:
Why don’t we put a comma in the year (that is, 2008, not 2,008)?
Oh, and –1 for missing the placement of the question mark. 
With the possible exception of this particular decade, years (and house numbers) are normally pronounced differently from “regular” numbers. For example, 1978 is nineteen seventy-eight; 1,978 is one thousand, nine hundred seventy-eight. 1900 was nineteen hundred, not one thousand nine hundred. 2259 will be “twenty-two fifty-nine.”
I don’t know how we’ll pronounce the years once they get into quintuple digits.
Right, but how’d it get that way? Years are a count from some event—it’s been five years since The Big Guy was born. It’s been 142 years since the Grand Shuffle. Can you believe it’s been 999 years since The Event? Someone should write a song about a festival ‘round this year. OMG, it’s been, like, two thousand and eight years! When it’s all written out on a wedding invitation or whatnot (i.e.g., in the year of our Lord, nineteen hundred and seventy six) the numericalishness really stands out.
I get how we think of it now, and wouldn’t be surprised if it’s just one of those conventions that just is. But I thought there might be some explanation that was intentional (those busybodies at Trent thought commas looked like devil’s horns), typographic (Guttenberg’s press wouldn’t line up correctly after Tackleberry futzed with it), or some such practicality.
Oh, about no comma in a four digit number. A few of my clients’ style manuals do that. They’re (the clients) international hodgepodges, if that sheds any light on it. Maybe it’s a Hodgepodgian thing?
The numerical character of years is also very evident if you write them in Roman numerals. where you have to do some math (adding to and subtracting from the nearest round number).
This is nitpicky, but I’d like to point out that ordinary numbers are not adjectives either, grammatically speaking. In a sentence like, “I saw four cars driving down the street,” the word “four” is not an adjective modifying the noun “cars,” the way (for example) “big” would be if I were talking about big cars. Number names are a category of words on their own.