The Clock app takes advantage of some features in the gaia build system to do some optimizations:

• Apps can define an app-specific Makefile in their directory. This is run before the rest of the general Gaia build steps. Email uses this to create a directory in gaia/build_stage/clock and runs some optimizations around JS and CSS concatenation.
• The Gaia build system knows to use gaia/build_stage/clock to do the rest of the Gaia build steps because Clock specifies the "dir" in the gaia_build.json in this directory.
• Since the shared resources referenced by Clock are not listed in the HTML, but as CSS @imports or via JS module dependencies, the Clock Makefile runs clock/build/make_gaia_shared.js to generate a gaia_shared.json file in the gaia/build_stage/clock directory to list out the shared items use by the clock app. gaia_shared.json is used by the general Gaia build system to know what shared resources to keep. The Gaia build system also does HTML file scanning to find shared resources too.

For DEBUG=1 builds, the clock source directory is used as-is, and the shared resources are magically linked in via the Gaia build system.

If you want to give snapshots of builds, say for UX reviews, you should use the contents of the gaia/build_stage/clock directory as it will be a fully functional snapshot.