[3SidedSquare]

You'll need JavaScript for this, (html is not a programming language).
You just need to be a little creative with html input boxes.
I think this is what you're looking for: Input tags have the following attributes
type="date" (used for entering a date)
Button tags can have the following attributes:
onclick="javascriptfunction()"
In javascript, you can get the value of a input element using
tag.val()

Good luck!