Its the date issued , some may have this.
My passport was issued more than 10 years ago - can I enter the EU?
No. Some holidaymakers have been caught out at airports because their passports were issued more than 10 years ago. Up until 2018, the UK government allowed for unspent time on passports to be added to the new passport when renewing, meaning some UK passports are valid for more than 10 years.
This didn’t matter while the UK was in the EU because passports were valid up to and including the expiry date. But you will now be denied entry if your passport issue date was more than 10 years ago.