ERROR: DefineIndex: index function must be marked iscachable

With version 7.2, when I tried to create an index using the built-in
funtion date(), I got an error:

ai=> create index stl_date on stl(date(stl_trxdate) );
ERROR:  DefineIndex: index function must be marked iscachable

What does this mean?  I could do so under version 7.1.2