People with more experience in styling might be able to provide a better answer, but - using css or javascript, you should be able to remove this class or override it and then replace it with your own:

Alternatively, there are many custom datepickers available in the marketplace that you could use, but I assume you'd rather stick with the standard Mendix functionality.

Hello Simon,

you can change the content of the icon by override the class like below.

for me, I used mIcons module to change the icon with different one,

.mx-icon-filled.mx-icon-calendar::before {
    content: "\e959";
}

Hello Siman,

try with Advance date picker or Customizable Date Picker if that will help, else go with CSS changes.