Dates and rounding problem

[david at anon.nu]

I'm working on an app that does a lot of date calculations. I need to be 
able to round the seconds to exact days, but when I use convert like this:

    convert date to short date
    convert to seconds

I get the date rounded to sometimes two, or sometime 3 hours after 
midnight (depending on which date I use). I'm on linux - and i figure it 
is something to do with the locale / timezone, but still - why the 
different rounding errors?