Exclusion constraints with time expressions

Hi

I am using 9.1.6, and I've set up a partitioned table as described in the
manual, with partitions based on a timestamptz column called 'time'. The
exclusion constraints work nicely when I select ranges of times with
literal constants. But why would a WHERE clause like the following not
benefit from exclusion constraints?

time > TIMESTAMPTZ '2012-11-03 00:00:00Z' + INTERVAL '24 hours'

Isn't that expression on the right reducible to a constant up front?
Obviously I can use a host language to do the arithmetic and provide a
constant, but I am curious to understand why that wouldn't be
constant-folded.

Thanks
Thomas