Calendar thinks GMT is four hours ahead of EST instead of five

I'm planning a trip to Europe in March 2024. When I enter times for events in the UK or Iceland in Calendar, using Time Zone Support, the result is always off by one hour. If I give the time in Eastern Standard Time, then change it to Greenwich Mean Time (or United Kingdom Time), Calendar will register the time difference as four hours, when it's actually five hours. As a result, all of my calendar listings are incorrect in one time zone or the other. I'm using a MacBook Pro at home and an iMac at work -- it makes no difference to the problem. I've also tried adjusting my Date & Time settings, which is similarly unhelpful.