By days I assume you mean dates. You will need to transform the date into a number of milliseconds from an epoch. In FormCalc this is done using the Date2Num funnction. Once you convert both dates then you can subtract one from the other to get a difference then change it from milliseconds to whatever units you want.
This can be done in Javascript but it is more difficult.
I came across a sample where I did this before. Remember that this is FormCalc. The function returns the number of days since the epoch not the number of milliseconds (thats what Javascript does). Here is the command I used: