We're working on migrating the builds over to a new coordinated system that is not only faster but will have consistent timestamps for everything across all builds that start at the same time. It will trigger at set times, multiple times per day, rather than on demand as commits happen, but can still be set off manually if we want.
Other than the timestamps being more "even" in most cases, it should be transparent to users.
The timestamp on the individual images will be when the entire build run starts and not necessarily when that kernel build timestamp happens.