I'm interested in a license that says something like "After the 1st of January 2020, you may distribute this work under the terms of the GPL v3" (or some other open source license). I'm interested in developing software products and keeping a limited period of monopoly for commercial exploitation, but granting people open source freedoms far sooner than copyright law would put my works in the public domain.
I saw an existing question (Can I force my work to be open sourced in the future?), but that question is largely about the logistics of doing so (e.g. "I want to add value to users by guaranteeing that the work eventually becomes free and maintainable by volunteers, even if I get hit by a bus or I go bankrupt").
In this question I'm interested specifically in licenses that would grant any licensee the traditional open source freedoms at a specific point in the future. I'm wondering if there are any licenses already in use, considering the oft-repeated advice to try to avoid modifying existing licenses.
A closely related question: would such a license be GPL compatible (after the open source date; obviously not before)? Or would the additional historical restrictions prevent it being combined with other GPL works?
(I'm aware that using this sort of license implies that I am actually giving the source code to someone earlier than the open source period; if I just refrain from publishing the source until I want it to be open source, then there's no need for a special license)