Max function on Timestamp

I have a table in my database with a timestamp field and I can't use a
max function on it. You can do it with any other date types (date,
timespan, etc.) but not with timestamp.
Is there any problem on doing it? Why isn't it implemented in PostgreSQL
6.5.1? What would be the easiest way of doing it?
Thank you in advance,

                Esteban Chiner

P.D: There aren't also any "less than" or "greater than" operators on
timestamp...

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 Esteban Chiner Sanz        mailto (work): echiner@tissat.es
 TISSAT
 Av. Aragon, 30, 5ª planta  Phone: 96 393 9950
 Valencia (SPAIN)