Index scan vs. Seq scan on timestamps

List,

PG version is 7.4.2

I log apache hits to a postgres server. The table layout is as follows:

apachelog=# \d accesslog
Table "public.accesslog"
Column | Type | Modifiers
----------+--------------------------+-----------
id | integer | not null
ip | character varying(15) |
ident | character varying(200) |
auth | character varying(200) |
time | timestamp with time zone |
request | character varying(200) |
code | integer |
bytes | integer |
referrer | character varying(200) |
agent | character varying(200) |
Indexes:
"accesslog_pkey" primary key, btree (id)
"accesslog_time_idx" btree ("time")

Number of rows: approx: 530.000

If I want to count hits 30 days back this query uses an efficient
index scan:

select count(*)
from accesslog
where time between (current_timestamp - INTERVAL '30 d') and
current_timestamp;

Aggregate (cost=8294.40..8294.40 rows=1 width=0)
-> Index Scan using accesslog_time_idx on accesslog
(cost=0.00..8287.97 rows=2569 width=0)
Index Cond: (("time" >= (('now'::text)::timestamp(6) with time
zone - '30 days'::interval)) AND ("time" <= ('now'::text)::timestamp(6)
with time zone))
(3 rows)

while this query uses a less efficient seq scan:

select count(*)
from accesslog
where time between (timeofday()::timestamp - INTERVAL '30 d') and
timeofday()::timestamp;

Aggregate (cost=34966.56..34966.56 rows=1 width=0)
-> Seq Scan on accesslog (cost=0.00..34823.86 rows=57077 width=0)
Filter: (("time" >= (((timeofday())::timestamp without time
zone - '30 days'::interval))::timestamp with time zone) AND ("time" <=
((timeofday())::timestamp without time zone)::timestamp with time zone))
(3 rows)

Why does PG not use the index on the time column in the second select,
timeofday() has been cast to a timestamp after all.

Any insight much appreciated.

Regards
Per
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