Numpad: https://numpad.io/

It's a web-based notepad calculator, which means it's a notes app but it can evaluate inline calculations like

``` £300 in USD + 20%

09:00 to 18:30 - 45 minutes ```

I wrote the core of the calculator a few years ago, and I've just launched a big rewrite that supports

* document syncing * offline editing * markdown formatting * PDF and HTML exports * autocomplete * vim mode

Happy to hear feedback :)