I have a question, or maybe I noticed one bug on your website.
When I open it, page loads and it shows current market value of Bitcoin in USD with chart, but when I switch to any other currency (eur, chf, pound, etc) or when I click back to USD, there is no more chart.
If I want to get back chart I need to click below on buttons for 1D, 1W, 1M, etc.
Maybe this is made by design, but I would like to see automatic loading of charts with all currencies.

Suiggest :

• Provide the Link / Export

Info : for example i want proof or show someone about the date , i provide a link from your site :
for example a button like Generate Link or ...

Sample Link : https://bitcoinpricecalc.com/xun3s
or Another : https://bitcoinpricecalc.com/20210703

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there was another one .... i already forget about it

If you ask for it, you get it 
Nothing big, still...

I've looked at your API. I think that the date and timestamp used in requests/responses would be better to be UTC, so one doesn't have to guess your timezone and DST dates.
Also, if people will start using your API, you may have to restrict the number of calls/minute so you don't get flooded.

Why do you call it USDT price calculator?

You are showing cryptocurrencies prices in USD.

USDT, BUSD, USDC or any other stable coins are all the same, they are all just = 1 USD.

I would call it USD price calculator.. or crytpocurrencies price calculator.

It should be much bigger than this and there is still a lot of empty space on home page, but that is just my opinion, so you can ask other people for feedback.
Maybe something like this I posted below or play around more with colors and size.

  vs 
                               original                                                                                        my quick suggestion

Found a small bug in the historic prices:

• It looks like the amount of data that we could access, differs based on what currency we're viewing the data...
  
  • $ = Starting from February 2, 2012 onwards
  • € = Starting from September 12, 2013 onwards
  • ¥ = Starting from June 25, 2015 onwards

I don't know what sources you are using to fetch Bitcoin prices but you should mention them somewhere (I couldn't find any info about that), and using multiple sources mid value would be the best option.

There is a lot of blank white space on your website so I would definitely make those numbers for BTC and fiat currencies bigger, and date font size should be smaller.

I like date and calendar feature but oldest date I can find for historic Bitcoin prices is September 2013, so adding prices before that would be great.

Your site shows in the box, now, for example, for 100 BTC -> 4231184.00
With digit grouping it would be either 4 231 184.00, either 4,231,184.00, for easier reading.

It's interesting that in page title you did put some sort of grouping (although dot is usually the decimal point, but I know that it depends on the country/locale) 42.311,84

I would suggest making a button to jump back to the current date.
After looking at a historic date I was looking for a simple button to jump back to todays price but I had to reload the page

Found a small bug in the historic prices:

• It looks like the amount of data that we could access, differs based on what currency we're viewing the data...
  
  • $ = Starting from February 2, 2012 onwards
  • € = Starting from September 12, 2013 onwards
  • ¥ = Starting from June 25, 2015 onwards

Good job! I may myself use this. Here's some feedback:

• Add night mode.
• Add prices prior to 2013. (2009-2012)
• Add historical data for each month.
• Add lightning donations!

Nice site, I like the historical feature and that you have a simple API. Keep it up!