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We (non native English-speakers) are writing a paper and are wondering if the following construct is valid English:

Yesterday's assumption is no longer valid.

Specifically the apostrophe after yesterday (and likewise in today and tomorrow) brings up some debate.

Can anyone give a clear answer whether this is proper English?

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The apostrophe indicates possession.

Without an apostrophe you are indicating plurality.

Since the point you are trying to convey is that the assumption you made yesterday is no longer valid, the apostrophe is appropriate.

Yesterday's assumption is no longer valid.

It's kind of like saying "The assumption of yesterday".

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It's absolutely necessary, to show that you mean of yesterday. The alternative, plural of yesterday, is rare but does exist : "All our yesterdays have lighted fools/ the way to dusty death", Macbeth.

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    Upvote for quoting Shakespeare :-) – Phil Whittington Feb 28 '12 at 1:04
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    Absolute upvote for quoting Shakespeare :) – user20874 May 5 '12 at 1:22
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Pedants would have it that 'yesterday' is not a person and therefore cannot own anything. They would suggest that the sentence be re-worded. "The assumption made yesterday... ."

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    Linguists would counter that genitive constructions are used for many things other than pure possession, and that the pedants’ point here is irrelevant. – Janus Bahs Jacquet Aug 31 '13 at 9:17
  • Genitival construction did not appear to me to be of concern in the question raised. I apologise for any misunderstanding I may have caused. – Tony Aug 31 '13 at 9:51

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