Harvest date and expiry date: which one matters

Almost every bottle of olive oil carries an expiry or "best before" date. Far fewer carry a harvest date, and the harvest date is the one that actually tells you something.

Why the expiry date misleads

An expiry date is normally counted from the date the oil was bottled — often 18 to 24 months after. The problem is that olive oil is not necessarily bottled as soon as it is pressed.

Oil can sit in bulk tanks for months, sometimes over a year, before it is bottled. The clock starts when it enters the bottle, not when it left the tree.

That means a bottle with an expiry two years away can hold oil pressed three years ago. The date is not lying — it is simply measuring the wrong thing.

What a harvest date tells you

Olive oil does not improve with age. Unlike wine, it declines from the day it is pressed. Polyphenols diminish, flavour fades, and oxidation creeps forward even in a sealed bottle.

A harvest date tells you how far along that process is. It is the only date that relates to the oil's actual condition.

Why most bottles do not show it

In most markets a harvest date is not required. A producer who presses and bottles quickly has a reason to show it; one whose oil sat in a tank for a year does not.

That in itself makes it useful. Its absence is not proof of anything bad, but its presence is a meaningful signal.

How long olive oil actually keeps

• Unopened, kept cool and dark: good to the date on the label, and at its best within the first year after harvest
• Once opened: use within about two months for the best flavour
• Once opened and badly stored — bright, warm, frequently opened: considerably less
• Oxidised oil is still safe to eat, but the flavour and polyphenols you paid for are gone

How to check at home

Smell the bottle. Oxidised oil smells of crayons, cardboard, putty or stale nuts. It tastes flat and slightly greasy, and it leaves no pungency in the throat.

If that is what you get, the bottle is past its purpose — whatever the date says.