So I wondered when the #301DaysOfCode challenge would end because I started yesterday and built a small add/subtract days calculator.
As it turns out you can easily "add" days to a date using .setDate() but you better remember that its actual purpose is to set the day of the month rather than sum days. Both of the following options seem to work well:

• "Dirty" but short:

resultDate.setDate(days + 1);

• "Proper" solution:

resultDate.setDate(resultDate.getDate() + days);

• MDN Date
• "Answer to 'Add days to [JS] Date'", StackOverflow

Side note: on a terminal you can just use things like:

date --date="301 days"
date --date="301 days ago"