It would not be open source for the duration of the time-bomb, clearly, that's the point of creating an exclusivity period. After the time-bomb, it's undeniabl

It would not be open source for the duration of the time-bomb, clearly, that's the point of creating an exclusivity period. After the time-bomb, it's undeniably open source: users have all the freedoms granted by open source and they have the option to release their derived version with a zero days time bomb, effectively equivalent in all respects to GPL. What GPL does not grant, and such a license would, is the ability to create a non-zero commercial exclusivity period, during which the software reverts to commercial-with-source-available status.