So then it's not a bug. The instance is defined in
Data.Time.Format.Parse, and Data.Time.Calendar doesn't import that
module.

This, however is a bug, I think:

Prelude> import Data.Time
Prelude Data.Time> read "2011-10-10" :: Day

... no instance for (Read Day) ...

Prelude> :m +Data.Time
Prelude Data.Time> read "2011-10-10" :: Day
2011-10-10


Erik