The best alternatives to Watzon are Team Time Zone, GMT NOW, and Time Travel. If these 3 options don't work for you, we've listed a few more alternatives below.
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GMTNOW is a progressive web app for timezone conversion. We have used NUXT and Vuetify for the project and backend runs on MongoDB. Since this is PWA you can access the same via https//gmtnow.app and also install the same onto to mobile, when can then onwards work even without internet connection.