If you have dates and want to know the new dates after certain days, you can add days directly to the dates. To get the date format you want, please combine with the TEXT Function.
=TEXT(A2+300,"MMM DD, YYYY")
For example, 300 days after January 15, 2016 is November 10, 2016.
To have the dates before certain days (e.g., 300 days), you can use the same formula except to use "minus".
=TEXT(A2-300,"MMM DD, YYYY")
Where "MMM DD, YYYY" returns month name with 3 letters. If you need the full name, please use "MMMM DD, YYYY".