%_buildtime is not an rpm macro: built with it alone and no SOURCE_DATE_EPOCH,
rpm 4.14.3, 4.16.1.3 and 4.19.1.1 all stamped the current time. The build time
and build host the release verifier checks come from SOURCE_DATE_EPOCH with
use_source_date_epoch_as_buildtime, and from _buildhost -- both honoured on
every one of those versions and on rpm 6.0.2, where the package also builds
and verifies unchanged.
Vinícius Ferrão