Do you have to register it with the copyright office as soon as you create it, or could you create it, register it 10 years later, then immediately sue someone who copied it?
However U.S. copyright law does have a statutory limit on how retroactive registrations can be. If you file a registration within 90 days of first publication, the registration is retroactive to the date of publication; but if you file more than 90 days from publication, registration is only effective from the date of registration.
Fwiw, even with unregistered works you have some options available to you, just not damages. For example, you can file a DMCA takedown notice.
An exception is foreign works, whose owners can file a suit without first registering, since treaty obligations don't allow the U.S. to subject foreign holders to "formalities". However foreign holders are limited to recovering "actual damages" in such suits, and need to register to be eligible for per-copy statutory damages.
In general, copyright registration is a legal formality intended to make a public record of the basic facts of a particular copyright. However, registration is not a condition of copyright protection. Even though registration is not a requirement for protection, the copyright law provides several inducements or advantages to encourage copyright owners to make registration. Among these advantages are the following:
• Registration establishes a public record of the copyright claim.
• Before an infringement suit may be filed in court, registration is necessary for works of U.S. origin.
• If made before or within five years of publication, registration will establish prima facie evidence in court of the validity of the copyright and of the facts stated in the certificate.
• If registration is made within three months after publication of the work or prior to an infringement of the work, statutory damages and attorney’s fees will be available to the copyright owner in court actions. Otherwise, only an award of actual damages and profits is available to the copyright owner.
• Registration allows the owner of the copyright to record the registration with the U.S. Customs Service for protection against the importation of infringing copies. For additional information, go to the U.S. Customs and Border Protection website at www.cbp.gov/.
Registration may be made at any time within the life of the copyright. Unlike the law before 1978, when a work has been registered in unpublished form, it is not necessary to make another registration when the work becomes published, although the copyright owner may register the published edition, if desired.
>However, registration is not a condition of copyright protection.
>Before an infringement suit may be filed in court, registration is necessary for works of U.S. origin.