Versions are often used to convey information about how different a particular version of the software is from the previous one. So for example, if you upgrade from 1.0 to 2.0 of Acme, that's a major upgrade, in theory carrying major changes.

An upgrade from 1.0 to 1.1, on the other hand, is a minor upgrade and in theory carries minor, incremental changes and bug fixes.

This would be difficult to represent in a date format, though you could use a mixture. For example, v1.0.YYYY.MMDD