The hydrant:year is NOT the year the hydrant was placed there. It's the year of manufacturing. 

Since it's not in the wiki what to do with that information, again to be consistent I "invented" as allowed by OSM main principle - a new tag for this namely the fire_hydrant:year 

if you know a better one I'm open for suggestions but I think without any further wiki information on manufacturing year it's as close and consistent as can be.

but it's NOT the start_date, as that i didn't see when I passed by and made a photo of this hydrant.