One reason is because unix time is not actually seconds since the epoch. It doesn't include leap seconds (a leap second is added about every 1½ years). Currently unix time is about 30 or so seconds behind "Number of seconds since (unix) epoch".
No, you can't reliably calculate it forward or backwards. To figure out the offset for dates in the past, you'd have to store the list of when leap seconds are added. This system is hardly a "just the number of seconds since epoch", but now have a arbitrary data attached. The official group that figures out leap seconds can't predict things more than 6 months in advance. How are you going to figure out in advance what's going to happen in 2 years time?
