What is "era format"?
 

ERA_D_FMT is in "era format" apparently. But I don't see any description of what "era format" is. Looks like some kind of bitfield?

Edit:

Think I found the info here.

Yes, ERA_D_FMT is a string used to describe how the date escape sequences (the same ones used by strftime()) will display the date in your locales era.

The era itself is usually the Christian era “AD” although you could just as easily create a BC era. In the real world, I believe there are certain locales which use the BE (Bhuddist Era) or AH (Islamic era) in their dates. You can find details in the glibc locale files. For example, look at the LC_TIME section of this file to see how the Bhuddist era is implemented and displayed in the Laos locale.