The simple way is to post a picture of you and the widget to a site like Flickr.
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.
Both the above provide evidence you had access to the widget before January 10th, but neither prove you owned 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).
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.