Conversion from Unix timestamp to datetime?

I'd like to be able to convert between times specified in Unix timestamp
format and SQL datetime values without going through the slow and rather
error-prone process of converting to a human-readable date notation.

I see how to get a datetime value returned as a Unix timestamp:

    tgl=> select date_part('epoch', 'now'::datetime);
    date_part
    ---------
    900355156
    (1 row)

But I don't see any way to supply a value in the form "900355156"
and have it get converted to datetime.  Is there a way to do that?

BTW, it seems like the standard data type 'timestamp' ought to be
good for this, but it isn't as far as I can tell ... it hasn't got
any working conversions to/from integer either.

            regards, tom lane