Writing Dates in Dialogue

Just the way you wrote it.

The rules for numbers is, generally, anything below ten is written out, anything above is written in numerals.

In dialogue I would write it out, rather than use numbers (Twenty-first Century).

Just did a quick web search. Basically, many of the sites agree that, with dialogue, the rules are thrown out the window.

Generally, they suggest writing out all numbers until it starts looking silly, then start tossing in numerals.

Which, I guess, is an odd way of saying that it's all about style, and how it looks on paper (or on the screen, as it were). My inner grammarian cringes at such fast and loose rules, but there it is.

Do what looks best.

One reason for writing out numbers in dialog is to show how your character actually speaks them. Of course, some numbers offer more scope than others, but it's an opportunity for voice and characterization.

"I've lived here since 1995." - Would your character say "ninety-five" or "nineteen ninety-five"?

"This paper is due on May 10." - Would your character say "May ten" or "May tenth"?

"We need to be there by 3:30." - Would your character say "three thirty" or "half past three"?

In dialogue, I would write it out. "Twenty-first century."

When to write numerals as words

This issue has caused me more migraines than most when it comes to my writing. I always felt bound by that "under ten, write it out" rule until I saw a grammar and style book that recommended using words for numbers up to twenty and yet another book that recommended going all the way to one hundred. Clearly, the jury is out, even among the experts.

I'd like to suggest what works for me: As I am reading a novel, I pay attention to how numbers are handled. For example, I had been using numerals for times of the day, but then I noticed in print they were always spelled out. 1:30 = no; one-thirty = yes. I must admit that the words look better on the printed page. I'm beginning to think numerals are mainly for academic papers (research discussions, etc.)

In any case, be sure to remember one hard-and-fast rule: You must use a word for the number if it comes at the beginning of the sentence:
21st century technology is confusing for many older people. No
Twenty-first century technology is confusing... Yes
(And by the way, everyone should have a reference larger than S & W. Check out the latest edition of The Gregg Reference Manual by Wm. Sabin. It's over 600 pages (or is that six hundred...), but has an excellent index and I found his answer to the "number with century" problem in about 10 seconds: Use words.)

One final thought. Maybe it's best to do what you think looks good and leave something for the editors to correct. They need to feel the love, too, you know.

I would write "Twenty-first century" in dialogue, too.