The site is nice and smooth, but it is very basic. I will give you some suggestions:

 - Add more platforms to compare prices.
 - Add the possibility of printing the price, time and amount.
 - Add historical data.
 - Add a specific title and track the title's movement in the charts.
 - Add more trading pairs.

More can be added, but you need the night mode now and quickly.

Nice! Although longer name than preev, it makes more sense.
I'll throw in also a suggestion: please consider digit grouping for bigger numbers.

Also, right now it's just a competitor for preev with the (undeniably) nice touch of historical data. Not bad, but you have the tools for more.
Maybe for medium-long term you'd also add more fiat currencies (maybe from xe or similar) and more (all?) crypto coins that can be traded, at least for spot price. Then the website will become important.

Interesting website and the domain name is quite good imo.

I made something similar some months back, but it doesn't have historical data.

I will make a suggestion. I think you could add a chart below your calculator, for the selected currency, as it will show the historical data as well, making it easier to navigate between dates.