What is the oldest code written still running?

Consul writes: This is something I've been thinking about lately, and I think it would make for an interesting exercise and debate. What is the oldest piece of code that is still in use today, that has not actually been retyped or reimplemented in some way? By "piece of code", I'm of course referring to a complete algorithm, and not just a single line. So, does anyone think there's a routine out there that has survived the ages simple by being copied around?