iscacheable for date/time?

I'm looking at pg_proc.h to adjust the cacheable attribute for date/time
functions. Can anyone recall why the interval data type would have been
considered non-cacheable? I didn't make internal changes to that type,
but istm that it should be cacheable already.

For timestamp and timestamptz, I've eliminated the "current" special
value which afaicr is the only reason timestamp had not been cacheable
in the past. Are there any functions which should *not* be considered
cacheable for those types? Apparently the _in() and _out() functions
should not be? Everything else is deterministic so would seem to be a
candidate.

Comments?
                       - Thomas