Why is the expiration date on my smoke alarm shorter than my warranty period?

Smoke detectors with a 10-year lithium battery must have an expiry date on the detector. This is laid down in the European Standards (EN Standards). Every brands has to comply to this rule and thus you will always find an expiration date on that kind of detector. The expiration date is always the production year plus 10 years. But a smoke detector almost never reaches the customer in a short time after production, so the expiration date will never actually be the full 10 years later. This expiry date is therefore a "soft date" that you can ignore. The warranty is only valid from the date of invoice and the battery can be used for 10 years after you active it.