This is a very incomplete answer, but I'm posting it in the hopes that someone else may be able to build on it.
I opened Inspector in Firefox and used it to determine the class for the select-all-images-for-date checkbox, and then entered this in the console:
var a = document.getElementsByClassName("R4HkWb");
for(var i=0; i<a.length; i++) a[i].click();
It almost works, but it has some severe limitations:
- It only selects images for dates that have more than one image. You have to go back and select images where the date has only one image.
- It only selects images for dates that are currently visible, so you have to repeat the command as you scroll down the page.
- I don't know how permanent that class is, you may have to dig into the page with the inspector to find the new class if it changes.
- If images in a date are already selected, running this command inverts the selection, deselecting them.
Again, I'm not posting this as a complete answer, but as a partial answer that will hopefully get someone closer to a full solution.