Should I assume your front matter field for dateAdd is a Unix time stamp in quotes/string? time converts the timestamp string into a time.Time. So, if the front matter key-value is a Unix time stamp, first format it through time then access .Month, which, by the way, returns the string (e.g., June) and not the month number (from what I can tell testing locally).
Maybe this will help get you what you want?
Let’s say you have addDate: "1489276800" in your front matter:
{{$time := time (int .Params.addDate)}}
<span>{{$time.Month}}</span>
=> "March"
Or are you looking for month number since you mention the “index of the month?”