What do AC and DC mean in years?

I'm curious, could you please explain what is meant by AC and DC when it comes to the context of years? It seems like an unusual pairing of terms, and I'm wondering if there's a specific meaning or application behind it that I'm not aware of. Is there a particular field or area of study where these abbreviations are commonly used in reference to time periods? I'd appreciate any insight you can provide on this matter.