extract epoch and index scanning

Can someone explain why the latter query uses an index scan while the
former uses a sequence scan?  I have tried all sorts of casting in the
first case but I cannot get the index scan.

Cheers,
Randall

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data=> EXPLAIN ANALYZE SELECT acqtimestamp FROM aux_datarecord where
acqtimestamp between (extract(epoch from timestamp '2004-02-21
22:39:57+00') - 50716800)::double precision and (extract(epoch from
timestamp '2004-02-21 22:43:52+00'::timestamp) - 50716800.0)::double
precision;
NOTICE:
QUERY PLAN:

Seq Scan on aux_datarecord  (cost=100000000.00..100548707.00
rows=477476 width=8) (actual time=114670.09..114670.09 rows=0 loops=1)
Total runtime: 114670.23 msec

EXPLAIN

data=> EXPLAIN ANALYZE SELECT acqtimestamp FROM aux_datarecord where
acqtimestamp between '1026686397'::double precision and
'1026686632'::double precision;
NOTICE:
QUERY PLAN:

Index Scan using aux_datarecord_acqtimestamp on aux_datarecord
(cost=0.00..3.54 rows=1 width=8) (actual time=0.66..0.66 rows=0
loops=1)
Total runtime: 0.83 msec

EXPLAIN
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