A client’s passport was issued on 21 June 2017 and expires on 20 June 2027. They want to travel from 15 December 2026 to 5 January 2027.
Six months from arrival is 15 June 2027, before the expiry date: it passes. Three months after leaving the Schengen area is 5 April 2027: it passes. Issued in the last ten years: on arrival the passport is nine and a half years old, so it passes. Six months after the trip is 5 July 2027, after the expiry date: it fails.
So for a client from outside the EU the passport works for a Christmas trip to Vienna or Rome, but not for a destination that counts six months from the end of the stay. When the trip ends, the passport has 5 months and 15 days left; to keep six months after the trip, the trip would have to end by 20 December 2026. For those destinations, the client should renew before you book.