I made a date time calculator tool with six different functions that I gave an old-school terminal-based look-n-feel. It has a few unique features that I have not seen on other similar web sites before. I have it try very hard to allow for a wide variety of inputs such as just a date or just a time; a three/four letter timezone, such as EDT or CEST as opposed to having to remember something like America/New_York (which it can also handle). Another unique feature would be adding or subtracting two durations. For example, adding 3 days 5 hours 14 minutes to 2 days 23 hours and 51 minutes. Another nice function is the ability to convert from one group of durations to another: 386 hours 24 minutes 36 seconds to just days hours minutes seconds.

There is also an example button for each function so that you can see how it works.

If you like find this useful, there is also a GitHub link to the CLI equivalent at the bottom of the page.

https://datetimemate.com/