Cannot Format "Mirror" Column Based on Timeline to Show Year

I have a board with a “Mirror” column that shows me the earliest of the dates in a subitems column.
Problem is that since things last for more than a year, i now have no way of knowing if my projects have been around for a month, or 13 months:
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Now, I see that in a regular “date” column i can check a box to show the year (which you almost always want in a date). But if its a “mirror” column, no such option exists.
How can the year be added to the date format of such a column?