When you can't find your passport or cannot access it temporarily, it may still be possible to calculate when the travel document will expire. A passport's expiration date is determined by its date of issuance and your age at the time your application was processed.Step 1. Find out when your passport was issued.Try to remember whether you got the passport for a specific international trip, and how many months before the trip you received your passport.hen you can't find your passport or cannot access it temporarily, it may still be possible to calculate when the travel document will expire. A passport's expiration date is determined by its date of issuance and your age at the time your application was processed.