Pretty neat!<p>Some UI suggestions:<p>* When adding a custom format, allow for an "X" to remove it as well.<p>* Clicking on any of the output formats should automatically copy the text to the clipboard. I like that it selects it, but it also selects the label as well (at least on FF). A straight copy would be more useful.<p>And some date/time suggestions from someone who's had to deal a LOT with dates and times. I realize you're using libraries for a lot of the heavy lifting, and maybe some of the ideas below aren't what you want to do with it, but they are real needs:<p>* Allow setting a custom epoch for both input and output.<p>* Allow setting a location other than the current location for output. Likewise, allow enabling/disabling of daylight savings for the user-provided location.<p>* It's currently impossible to represent any date before Jan 1, 100 AD.<p>* Non-Gregorian calendar support would be really, really nice, too (e.g. it's currently 4718 AH using the Chinese calendar and 1442 in the Islamic calendar).<p>To date I've just been using <a href="https://www.epochconverter.com/" rel="nofollow">https://www.epochconverter.com/</a> for a lot of my day-to-day work, but I like your input more. You may find some ideas there as well.