There is no time difference between Frankfurt and Berlin. Both cities use Central European Time (CET/CEST, UTC+1/+2). When it's 12:00 PM in Frankfurt, it's also 12:00 PM in Berlin.
Both cities observe daylight saving time on the same schedule, so they remain synchronized year-round.
Any time works! Since both cities share the same timezone, you can schedule calls during standard business hours (9:00 AM - 6:00 PM) without any time conversion needed.
Frankfurt and Berlin are Germany's two most important cities for business and government. Frankfurt is the financial capital with the European Central Bank, while Berlin is the political capital. High-speed trains connect them in about 4 hours.