Having trouble with Interval computation

I'd like to get the difference between two timestamp columns expressed as an interval with units "year to month."  I believe the standard way to express this is as:

   (timestamp1 - timestamp2) year to month

But this gives me a syntax error at "year".  I also can't find a way to cast this to the desired interval units.

What is the proper way to express this in Postgres?  Will the standard form be supported at some point?

Thanks very much,

Jim Ballard
jballard@netezza.com