Haskell time library: Weekday and Month

I recently wanted to move from old-time to time in my bibtex package, but I was no longer so enthusiastic about it, when I became aware, that for instance weekdays and months are handled as plain numbers. Wouldn't it be better to make them enumerations or newtypes (if efficiency is important)? For a complex matter like coping with time it would be nice to have precise types in order to know, what can be converted to what given what context. In case of weekdays and months a custom type would also make it irrelevant for the user, whether you count days and months starting from zero or one.