i am trying to write my first bit of javascript for my google sheet

You never return a value from myDays. So days = myDays(date2,date1); will result in days being undefined.

When we look further gradient = followers / days; will result in gradient always being NaN (not a number). Since something divided by undefined will always yield NaN.

To solve the issue add a return statement.

function myDays(date1 , date2 , followers1 , followers2) {
  const oneDay = 24 * 60 * 60 * 1000; // hours*minutes*seconds*milliseconds
  const firstDate = new Date(2008, 1, 12);
  const secondDate = new Date(2008, 1, 22);
  const diffDays = Math.round(Math.abs((firstDate - secondDate) / oneDay));
  return diffDays; // <- return the difference in days
}