The year is part of the copyright notice, not the license. It's important to understand that copyright notices are not very important now, after the widespread international ratification of the Berne Convention. Per the Berne Convention, copyright is automatic when a work is created. You cannot accidentally remove or invalidate your copyright by incorrectly marking your copyright. It does serve a function (described below), but that function is very different from creating or ensuring your copyright.
In the United States, a copyright notice should be made in compliance with 17 U.S. Code § 401:
(b) Form of Notice.—If a notice appears on the copies, it shall consist of the following three elements:
(1) the symbol © (the letter C in a circle), or the word “Copyright”, or the abbreviation “Copr.”; and
(2) the year of first publication of the work; 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 year date may be omitted where a pictorial, graphic, or sculptural work, with accompanying text matter, if any, is reproduced in or on greeting cards, postcards, stationery, jewelry, dolls, toys, or any useful articles; and
(3) the name of the owner of copyright in the work, or an abbreviation by which the name can be recognized, or a generally known alternative designation of the owner
While 17 USC 401 says that you should include a single year, the FSF (whose advice has presumably been vetted by at least one lawyer) recommends that for software with multiple releases, you include all applicable years:
Whichever license you plan to use, the process involves adding two elements to each source file of your program: a copyright notice (such as “Copyright 1999 Terry Jones”), and a statement of copying permission, saying that the program is distributed under the terms of the GNU General Public License (or the Lesser GPL).
The copyright notice should include the year in which you finished preparing the release (so if you finished it in 1998 but didn't post it until 1999, use 1998). You should add the proper year for each release; for example, “Copyright 1998, 1999 Terry Jones” if some versions were finished in 1998 and some were finished in 1999. If several people helped write the code, use all their names.
For software with several releases over multiple years, it's okay to use a range (“2008-2010”) instead of listing individual years (“2008, 2009, 2010”) if and only if every year in the range, inclusive, really is a “copyrightable” year that would be listed individually; and you make an explicit statement in your documentation about this usage.
Based on this advice, it is perfectly valid to include multiple years to represent that the work includes work released during multiple years.
So what does a copyright notice actually do?
In the United States, the modern purpose of the copyright notice is to nullify a claim of innocent infringement. An infringing distributor cannot successfully claim they didn't know that the work was under copyright if it was marked by a prominent, correctly-formed copyright notice:
(d) Evidentiary Weight of Notice.—
If a notice of copyright in the form and position specified by this section appears on the published copy [...], then no weight shall be given to such a defendant’s interposition of a defense based on innocent infringement [...]
I am not sure what's the worst that could happen if you failed to correctly identify all the release years that apply to a copyrighted work. Possibly, it could be easier for someone to claim innocent infringement in the United States (SPECULATION: especially if the incomplete date(s) given would suggest an expired copyright). Other possibilities may apply in other jurisdiction. However, in most nations (i.e., all that signed the Berne Convention), this would not negate your copyright in any way, since the Berne Convention makes copyright apply instantly, irrespective of any copyright notice.