In fact, the postcard itself doesn't show the headquarters of the United Nations, but Times Square, although it was sent from the headquartersof the United Nations. It's a complex in New York City. The complex has served as the official headquarters of the United Nations since 1952. It is located on Manhattan, on spacious grounds overlooking the East River.
The United Nations has three regional headquarters or headquarter districts. These are located in Geneva (Switzerland), Vienna (Austria) and Nairobi (Kenya). These adjunct offices help represent UN interests, facilitate diplomatic activities, and enjoy certain territorial privileges, but only the main headquarters in New York contains the seats of the principal organs of the UN, including the General Assembly and Security Council. All 15 of the United Nations' specialized agencies are located outside New York at these other headquarters or in other cities.
These stamps were issued some time ago... I see the last stamps says it's issued in 1991. But I like that last one especially. You probably can't see what is written in the middle of it so I'm copying: "Do unto others as you would have them do unto you." Thank you, Y.!