& "Days" added to the end of your calculation.
The calculation will have to return text.
Or you can just put "Days" as layout text to the right of your field. In some cases, this works best if you insert your calculation field as a merge field in to a block of layout text that contains "Days". In layout mode, it would look like this:
<<Your Calculationf Field Here>> Days
In browse mode, you'd see:
You can also select Currency data formatting and specify " Days" as the currency symbol.