I have trouble determining the first-use date of the following hypothetical example electronic product, which has a case and a circuit board inside. Both the case and the circuit board have the trademark on them.
Day 1. The assembled circuit board was ready.
Day 2. I showed the circuit board to audiences at a public conference, telling them this product would be ready for sale soon.
Day 3. The case was ready.
Day 4. The product (circuit board assembled in the case) was ready. It is shown at another conference.
Day 5. I gave a unit to a friend to show to a potential customer.
Day 6. I put the datasheet of the product on a web site.
Which day would qualify as the first-use date?
What specimen should I use, the picture of the circuit board or the case as part of the final product?
I believe the answer is none of the above. The first use date is almost always the same as the first use in commerce date. The first use in commerce date is the day that the product is sold in interstate commerce. This is an important detail, because, if you get it wrong your trademark may later be invalidated in court.
If you can identify the earliest date that you sold a product or service across state lines, this would likely be correct. However, a sale within a single state may still impact interstate commerce, and will likely qualify.
This becomes complicated quickly, for example, with services that are delivered for free over the Internet. The important part of identifying a first use date is identifying the related evidence, documentation, or proof. This evidence will likely be needed in any future litigation involving your trademark.