Reykjavik is 1 hour behind Warsaw
Best overlap: 9:00 AM to 6:00 PM in Warsaw is 8:00 AM to 5:00 PM in Reykjavik.
If you set up a recurring sync, 9:00 AM to 6:00 PM in Warsaw (8:00 AM to 5:00 PM in Reykjavik) is the natural home for it. Reykjavik is 1 hour behind Warsaw, but you still share a generous 9 hours of overlap, so scheduling is easy and there's room to move a meeting when you need to. Finally, Warsaw observes daylight saving and Reykjavik doesn't, so this window shifts by an hour for part of the year — recheck it each spring and autumn.