While this is indeed a monumentous occasion (
cough), you probably don't have to worry about your
software malfunctioning. See, the raw
timestamp date (seconds since the epoch) usually isn't represented in a place-value manner. If it's used, it's almost always stored as an
signed long integer in memory (4
bytes). The maximum value of this is 2147483647, so 1 billion should be
no sweat for most machines.
Also, when you hit the 1 billion mark depends on which timezone you're in...