sys-libs/glibc versions older than 2.3.5-r3 included the files in /usr/share/zoneinfo and the following binaries:
Newer (>= 2.3.5-r3) versions of glibc depend on sys-libs/timezone-data, but timezone-data doesn't depend on any specific version of glibc (this would create a circular dependency).
The consequence is that if you have an old glibc installed, you'll get tons of file collision errors when emerging sys-libs/timezone-data.
My suggestion is to add a warning to the timezone-data indicating the likely cause of file collision errors as well as possible solutions (ie update glibc or use COLLISION_IGNORE).
Steps to Reproduce:
1. emerge sys-libs/timezone-data on a system with sys-libs/glibc < 2.3.5-r3
added a blocker