Custom Calendar, Year as Measure or Dimension

Hi

I've created a custom calendar for my model. In this custom calendar, I have imported a a year field (called "Año" in spanish).

This field has a E (with green in the image). 

Also I've created a Year date with the built in functions from the date. (Field called Year, in the image)

What's the difference between this two fields? How do I change my imported field (Año) to be like the (Year) field?

Hope you understand it.

Thanks!