date.c: Remove occurrences of gboolean.

date.c: Remove occurrences of gboolean.

While this is surely one of the most innocent typedefs, it still
annoys me to have basic types like 'int' re-defined like this.
It just makes it harder to copy the code between projects, with
very little benefit in readability.

For readability, predicate functions and variables should be
obviously Boolean-natured by their actual *names*.