There are a few FOSS or FOSS-like licenses with time-based conditions.
One of the more common was the Netscape Public License (emphasis mine):
V.2. Other Products.
Netscape may include Covered Code in products other than the Netscape's Branded Code which are released by Netscape during the two (2) years following the release date of the Original Code, without such additional products becoming subject to the terms of this License, and may license such additional products on different terms from those contained in this License.
e.g. certain special terms apply only for the first two years.
Another license with a time provision is the TGPPL:
[...] to distribute or communicate copies of the Original Work and Derivative Works to the public, with the proviso that copies of Original Work or Derivative Works that You distribute or communicate shall be licensed under this Transitive Grace Period Public Licence no later than 12 months after You distributed or communicated said copies;
e.g. the license comes into force only after 12 months.
In general, time-based evolution is the essence of copyright protection and therefore the essence of FOSS copyright-based licenses (at least in the US) and the copyrights do expire after a period. (increasing longer)
Creative Commons also had a license e.g. the "Founders copyright" to effectively make a copyright expire after 14 or 28 years and therefore let the work "fall" in the public domain. It was used by O'reilly for a short while.
With all that said, I would advise against creating a new license.
Instead what you could consider is picking a license and adding an extra notice stating that your copyright claim will expire at some date and that the work will then be in the public domain. You likely want the help of a lawyer to draft this properly.