These answers were extracted from the book Patents, Copyright and Trademark, highly recommended. If you plan to buy one, notice that there's a newer edition than that I have.
Does a single date imply that the author claims copyright of the file from that date until eternity?
"The copyright lasts for the life of the author plus 70 years. However, under the following circunstances, the copyright lasts between 95 and 120 years, depending on the date the work is published:"
- "The work belongs to the author's employer under work made for hire
If a date range is used, and not kept up to date has the developer invalidated his/her own copyright beyond the date range?
No, simply the copyright will last from the last recorded date onwards.
Without some additional legal filing of some kind - does the copyright header provide any real legal strength of any kind? or are we all just fooling ourselves.
"Contrary to popular belief, providing a copyright notice or registering the work with the USCO is not necessary to obtain basic copyright protections. But there are some steps that can be taken to enhance the creator's ability to sue or stop others from copying:"
- "Place a copyright notice on a published work. (...) Placing this notice on a published work (...) prevents others from claiming that they did not know that the work was covered by copyright. This can be important if the author is forced to file a lawsuit to enforce the copyright, since it is much easier to recover significant money damages from a deliberate (as opposed to innocent) copyright infringer."
For clarification on the issue of registration, mentioned in the comments, the book states, under the same bullet list as the previous excerpt:
- Register works with the USCO. Timely registration (...) makes it much easier to sue and recover from an infringer. Registration creates a legal presumption that the copyright is valid and, if accomplished prior to someone copying the work, allows the copyright owner to recover up to $ 150,000 (and possibly attorney fees) without proving any actual money harm. (...)