Q. Why do we not use commas when writing years? What will happen in the year 10000?? I’m very concerned.
A. You have nothing to fear: Though commas are omitted from years that include no more than four digits, they’re recommended for years that run to five digits or more. So about eight thousand years from now (or just under 7,975 years as of April 2025)—when the year 9999 will have turned into 10,000 (as in 10,000 CE, or AD 10,000 if you prefer)—you can start adding commas to years to help you keep track of all those digits.
We don’t know why commas are omitted from four-digit years. But check back with us once commas are required again for everyday use. Maybe by then CMOS will have grown to more than 10,000 pages. If so—and assuming it’s still published in print and fits in a single volume (or two or more consecutively paginated volumes)—you should be able to turn to page 10000 or later to find out whether we’re still omitting commas from five-digit page numbers.
For more on years (including abbreviations like AD and CE), start with CMOS 9.36; for page numbers, see 9.63. For spelling out numbers versus using digits, start with 9.1.