I do NOT know the start_date, feel free to add it if you're a local and know the start date.

But the year - is the year of the manufacturing of this specific model/type by the manufacturer.

Since there's not yet on the wiki a tag for that - to be consistent with the other tags I added it as fire_hydrant:year

but that's definitely NOT the same as the "start_date"

for more information what start_date means see https://wiki.openstreetmap.org/wiki/Key:start_date

but again - if you're a local and know when this fire_hydrant was placed, feel free to add it - but don't replace the manufacturing year of that specific model with "start_date" as that's false information then.