As I recall, this is something we in fact used to have in gimp-2-10
branch and which we lost when switching from appstream-util to
appstreamcli validation tool (the later is now recommended, but it's
also less featureful, they don't even have a relax test!). So that was a
regression in the build system.
Note that it's not just not to have to put a date too early in our
AppStream metadata, but it's also that this way, we won't forget to
update it upon release (hence shipping with a wrong release data in
metadata). Indeed, when building a non-release version, date="TODO" will
pass the test, yet moving on to a point release version, the validation
test will now fail on a TODO date.
9368c8abdd