A dropdown to select between all timezones it would be awesome, but to start easy, I would add something like a checkbox "Display local time" using system's timezone. Just and idea! 
I'm far from an expert in React (actually i've never worked with it) but I see you use {entry.date.toLocaleString('en-GB', { timeZone: 'UTC' })} to display the date. I assume it's possible to configure the 'UTC' constant as a global variable. It can even be a setting on the SignalK's plugin config page, where user manually specifies "UTC", "UTC+1", or something like that. Just trying to find out the easiest way to solve this. I would write it myself and make a pull request, but i don't think I'm skilled enough :S
In any case, i have nothing to do tomorrow, so I'll try.
I'm far from an expert in React (actually i've never worked with it) but I see you use {entry.date.toLocaleString('en-GB', { timeZone: 'UTC' })} to display the date. I assume it's possible to configure the 'UTC' constant as a global variable. It can even be a setting on the SignalK's plugin config page, where user manually specifies "UTC", "UTC+1", or something like that. Just trying to find out the easiest way to solve this. I would write it myself and make a pull request, but i don't think I'm skilled enough :S
In any case, i have nothing to do tomorrow, so I'll try.