Notice is optional
Copyright notice is not required, rather it is a warning to the public that a work is indeed protected and (more importantly) by whom, so you can determine if the author is dead. The US law spelling out duration of copyright, 17 USC 302 says that
Copyright in a work created on or after January 1, 1978, subsists from
its creation and, except as provided by the following subsections,
endures for a term consisting of the life of the author and 70 years
after the author’s death
In the case of an anonymous work, a pseudonymous work, or a work made
for hire, the copyright endures for a term of 95 years from the year
of its first publication, or a term of 120 years from the year of
its creation, whichever expires first
The point is that creation and publication are distinct, and the law recognizes this: copyright starts from creation, not publication.
Since copyright duration is expressed in terms of a fixed period after the author's death, it does not matter exactly when a sub-part of a work was created, and giving a year is tradition but legally superficial. Except, of course, in a work for hire, where the author (the company) does not "die", and that is when the date becomes important.
The law of notices
There is also law pertaining to notices: 17 USC 401, which says
a notice of copyright as provided by this section may be placed on
publicly distributed copies
If you do decide to include a notice, then
If a notice appears on the copies, it shall consist of the following
one of which is
the year of first publication of the work; or in the case of
compilations, or derivative works incorporating previously published
material, the year date of first publication of the compilation or
derivative work is sufficient.
The notice does not define the duration of copyright, rather, if related to subsection (d) which says
If a notice of copyright in the form and position specified by this
section appears on the published copy or copies to which a defendant
in a copyright infringement suit had access, then no weight shall be
given to such a defendant’s interposition of a defense based on
innocent infringement in mitigation of actual or statutory damages,
except as provided in the last sentence of section 504(c)(2)
Basically, the copyright notice precludes an innocent infringement defense. So, following US law, the date of copyright to be included in the notice is the date of its first publication. The duration of copyright is determined separately.