Interesting. In 5.0aw/sp3, I tried this and some variations, too.

x={^1998/10/27 08:30:00 A}
SELECT * FROM c_tbl WHERE x = dt_tm && This selects the correct record
SELECT * FROM c_tbl WHERE dt_tm = x && This results in an empty data set.

>SELECT * FROM c_tbl WHERE dt_tm = {^1998/10/27 08:30:00 A}