Timee is a useful website for this. I use an app called Time There to help me find the right times to record Automators with David, it can only show two time zones at a time though. I have seen an app in beta but it’s very early and I’m not able to even say the name or the developer yet!
And while this isn’t the app you are looking for, I really like Klok as a widget to easily check the time elsewhere vs where I am.
If you set the time for any of the cities (or any others you add) then slide the red slider at the bottom – click the image to see it – they will all change in tandem. Available on any browser – and there’s a TimeAndDate app that has that slider feature too – $2.99.
My solution to this is always just to Google “11 am pdt to local time” or similar and let Google work it out for me. Not sure if that will work for you—I’m sure you could set up a shortcut or similar to do this from the share menu, for example.
For time zones I want to use frequently I have various shortcuts set up using the timezonedb API. I like this because I can reuse them to, for example, adjust times for OmniFocus.
And if you were working across the same multiple time zones frequently I would second @RosemaryOrchard’s Klok recommendation. In addition to showing the local time you can adjust the time in one of the time zones and the others update too.
I remember a Belgian company disappointed a lot of their customers once because of this, they didn’t realise Central European time and Central European summer time use different abbreviations, and Google didn’t say “oh, but Belgium is in summer time right now” and as such, the customers missed product releases and the products were sold out. One reason I prefer cities usually
Not that this adds to the conversation per sae, but something to keep in mind!
World Clock by timeanddate is a great app that should tick most of the boxes. They also have a Meeting Planner app that is great for international teams. It even keeps track of National Holidays and other non-working days.
The thing I like about it is that you enter a list of cities into the app, then tap on one city, and there’s a slider at the bottom. As you slide the time, the rest of the cities time moves. So, once you slide to your require time, the cities display all the correct time for you.
(sorry for the low res. I tried to use a video to GiF converter and it gives lousy result. Any good app to recommend for Video to GIF?)