Hi @mingala!
Where do you see that at? The alpha-group-signicance plots in the Moving Pictures tutorial exclude the Year category, just as you described above:
We are in the process of overhauling metadata, and we are going to support optional typing of metadata columns, which would allow you to declare the Year as categorical, but that hasn't dropped just yet.
In the meantime, you could trick QIIME 2 into thinking that column is categorical by adding some text to the values (e.g. Y2017 instead of 2017).
Let us know how it goes! 