date_trunc'd timestamp index possible?

I'm trying to create a index from a timestamp+tz field and want the index
to be date_trunc'd down to just the date

when i try to do a

create idxfoo on foo (date(footime));

i get a

ERROR:  DefineIndex: index function must be marked IMMUTABLE

and it chokes on when i try to use the date_trunc() function as well

create idxfoo on foo (date_trunc('day',footime));

ERROR:  parser: parse error at or near "'day'" at character 53

Any suggestions/workarounds (other than creating additional date-only
columns in the schema and indexing those???)

-d