Back in the (good old) days of British Caledonian, they had a regular flight to Tripoli. After landing there one day, the captain made his usual announcement, followed by "...there is a time difference between Britain and Libya - please put your watches back 2000 years."
Heard on Southwest last year: "Make sure you take all your personal belongings with you. If you do leave anything behind, it'll be on eBay tomorrow."
In the days of BEA, the Trident used to do the regular run to Moscow. On the return leg, the captain would sometimes come on and say "Ladies and gentlemen, we have now left Soviet airspace." Cue much relief, and another round of champagne.