When you talk about Date objects, do you mean datetime? Because a Date with no time compenent has tons and tons of real world uses that a unix time stamp would be inappropriate for.
Can you provide examples? It only seems useful to me for display purposes if you know you'll never need to worry about any specific local time zone (or if you will, then you know what time zone that is and know how to deal with it).
Just general calendar arithmetic. There are tons of cases where you need to do stuff like calculate "one month before", "one month after", "beginning of the year", ... --that's a mess with unix timestamps, but trivial with a good calendar library (or postgres, for that matter).
Well, yes, you can convert on input and/or output from/to unix timestamps, sure (and you can with postgres, too), but the point is that you have to convert first, you can't do date arithmetic on unix timestamps directly (because there is no fixed relation between unix timestamps and days).
