Clocker stacks every time zone you care about in a single menu-bar dropdown — give each one a custom label (“Mum”, “Berlin team”, “the SRE on call”) and pin a few to the bar itself. The killer detail is the timeline slider at the bottom: drag it forward and every clock updates in lock-step, so finding a 9-to-6 overlap across three continents is a five-second scrub instead of arithmetic. Free, open source, on the App Store too.