Adrian Johnson’s kids were given the biscuits following their trick or treating.
He shared his predicament on Twitter with a photo of the pack of McVitie’s ‘original homewheat milk chocolate digestive’, with a best before date of 15 May 1999.
Mr Johnson, from Leeds, posted: “Think I’ve just eaten a biscuit (from a packet given to our kids while trick or treating) that went out of date 18 years ago. Not joking.”
He replied with another tweet: “Don’t think this specific brand exists anymore. Is there a doctor in the house?”
The biscuits were described as “awful” leaving Mr Johnson with a “long-lingering bad taste”.
He told HuffPostUK: “I sniffed them, looked at the biscuit side, which was quite white rather than the usual brown-ish colour, and it was then that I checked the sell-by date.”
His ordeal amused thousands with his tweet being shared more than 2,500 times with poster poking fun at Mr Johnson.
One person noted “Jesus these biscuits are old enough to vote”, while another teased: “It’s only a best before - you’ll be fine! They might get better with age!”