How can I prove that I owned an object before a certain date?
<ol>
<li>I received a widget on January 1.</li>
<li>A law is passed criminalizing ownership of widgets.</li>
<li>It is an affirmative defense that the widget owner had it before January 10th.</li>
</ol>
<p>How can I prove that I owned the widget before January 10th? (Assume the current date is before January 10th, so I have time to create evidence.)</p>

<p>The simple way is to post a picture of you and the widget to a site like Flickr.</p>
<p>A more expensive way (but with rather more weight) is to get a Notary to certify they saw you and the widget on January 9th.</p>
<p>Both the above provide evidence you had access to the widget before January 10th, but neither prove you <em>owned</em> it.  For that, you would need a dated (possibly even notarized) bill of sale or similar - but if you acquired the widget via a gift, that won't work.  A signed witness statement from the giver would probably serve (and the statement could legitimately be created when you are prosecuted).</p>
<p>None of the above are unforgeable - but you don't need that.  Depending on how the law is written, you will only need to show ownership on the balance of probabilities, or you might only need to show reasonable doubt that you didn't own it on 9th January.</p>

<p>Unless there is a provision in the hypothetical statute that deems a particular form of evidence as conclusive or presumptive proof of ownership, there is nothing you can do ahead of time to absolutely be sure you will be able to prove ownership in court. Some real property registries are the closest I am aware of to providing this kind of certainty.</p>
<p>However, outside of those regimes, any evidence that is relevant and not excluded by rules of evidence would be admissible and would be taken into account when deciding whether you in fact owned the object on a particular date. This would include:</p>
<ul>
<li>testimony or affidavit of your own or of witnesses</li>
<li>documentary evidence (e.g. receipts, photos, contemporaneous notes)</li>
<li>other physical evidence of possession (e.g. if the item is a tool, artifacts produced by that tool pre-dating the date of interest, combined with expert evidence about the estimated date of production)</li>
<li>probably a bunch of other things that I am not imaginative enough to think of</li>
</ul>

united-states criminal-law evidence rules-of-evidence