# Which of “couldn't”, “can't”, and “doesn't” for 13/13/13?

I recently came across an English learner saying that 12/12/12 is the last time such a repetition of day/month/year (or month/day/year, or year/month/day) will occur in the Gregorian calendar for a long while "Because 13/13/13 wouldn't exist."

I don't know whether "couldn't", "can't", or "doesn't" would the best word to use, from a prescriptivist perspective.

Based on personal recollection, I assume "can't" would be suitable for a real object that is unable to do something:

My car can't start

Or you could use "don't" for something that's possible, but doesn't happen:

I don't travel via the tollway.

But the problem is that 13/13/13 is not like a car. As well as being an abstract noun, it's also a logical contradiction. Does that mean I should use a subjunctive?

Assuming that "couldn't" and "wouldn't" are subjunctive, this answer seems to suggest "couldn't" would be a better fit than "wouldn't", as it's an "Equivalent of "can" (possibility)".

Also, what would be the most common expression from a descriptivist, as opposed to a prescriptivist, perspective? Based on a google search of "13/13/13 * exist", it seems to be "doesn't" - does that sound plausible?

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I'd never say “13/13/13 couldn't/can't/doesn't/won't exist”, but instead would say, “13/13/13 is not a valid date representation.” – jwpat7 Dec 31 '12 at 2:58
Saying "X wouldn't exist" has an expectation of a condition "...if something else should occur" so it just doesn't make sense here. "couldn't" and "can't" are only slightly better because they both imply the possibility that they exist. "Because 13/13/13 doesn't exist" is correct, because it is a situation true or false of a calendar; it either exists or doesn't exist. – Mitch Dec 31 '12 at 3:09
@jwpat7 that sounds like something a programmer or a mathematician or a logician would say. (Then again, most people on Stack Exchange come here via Stack Overflow!) – Andrew Grimm Dec 31 '12 at 3:35
13/13/13 is not a valid date representation because it purports to represent a date which is itself invalid. There will never be a date (in our current calendar) that could be written in that form. – Fortiter Dec 31 '12 at 4:09
@Fortiter your point being? – Andrew Grimm Dec 31 '12 at 4:16

Just going based on what sounds right to my ear:

doesn't -- Preferred if I was just trying to state a simple fact. "13/13/13 doesn't exist". Much as I might say: "Little green men on Mars don't exist."

can't -- Preferred if I was trying to point out how the calendar system prevents such a date. "13/13/13 can't exist because it's an invalid date!"

won't -- Works as long as the date is in the future but sounds a little clunky. Wouldn't make sense to say this about 13/13/13 once 2014 rolls around, even though the date is equally invalid then as it is now.

wouldn't/couldn't -- Sound too wishy-washy, like they're entertaining the possibility that maybe the date could or would exist under the right circumstances.

Given your specific example, I think any of the first three could work, but I'd probably use doesn't.

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The main use of would is to express ‘unreal meaning’. Secondary uses are to express habit, volition and prediction. The expression of ‘unreal meaning’ is also the principal use of could, but it can also express possibility, ability and, less often, permission. The expression of possibility and ability are the main uses of can, which also has a less important use in expressing permission. Would and could are not subjunctive forms. Do, unlike the three preceding verbs, is not a modal verb, but a primary auxiliary verb used, among other things, to form the negative of lexical verbs.

To say that 13/13/13 wouldn't exist is to suggest that it is unlikely to happen. Since we know that it won’t happen, wouldn’t is an inappropriate choice, so we should say instead 13/13/13 won’t exist or 13/13/13 doesn’t exist, although it is more likely that the thought would be expressed as something like There will be no 13/13/13.

My car can’t start is an improbable thing for anyone to say, because, unless we are being particularly whimsical, we don’t credit cars with ability in quite the same way we do with humans. We would normally turn to the negative form of another modal verb, will, and say My car won’t start. This is an instance of using will, in its negative form, to make a ‘present prediction’, that is, to say that something is likely, or, in this case, unlikely, to happen now.

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A language prescriptivist would identify the grammar or spelling rules which apply, and then identify the choice(s) which are consistent with the rules. As all the choices are grammatically correct, none of the choices would present a problem for a prescriptivist. Try a logician.

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