Hello, I don't know whether this is a bug or a wanted behaviour. Since the last stable portage-version (2.1.2-r9) the timestamp in /var/cache/edb/dep/timestamp.chk ist not updated anymore, when you have synced. I used that file to check the time of the last sync for a karamba theme called ViewPortageX. Please fix it or tell me, from where I can the timestamp of the last sync the best way.
/usr/portage/metadata/timestamp.chk (or ${PORTDIR}/metadata/timestamp.chk in a more generic way)