[pdonis]

> The first day of the 1st Century is Jan 1, 1 AD.

That's not what the poster I originally responded to is saying. He's saying the 1st Century should start on a nonexistent day.

[antonvs]

You can make this work by having the 1st century start on the last day of 1 BC. Think of it as an overlap if you like; it doesn't really matter.

That allows for consistent zero-indexed centuries. It doesn't have any other practical consequences that matter.

[hgomersall]

No, I'm saying we ignore when it actually started and instead use the normal rules of counting to decide what to call the respective centuries.