This isn't that complicated.
In the field "TotalDaysOfTravel" add the following FormCalc script in the calculate event:
var datePattern = "MM/DD/YYYY" /if neccessary change the pattern to the format you use to display your dates
var startDate = Date2Num(BeginDate.formattedValue, datePattern)
var endDate = Date2Num(EndingDate.formattedValue, datePattern)
var numOfDays = endDate - startDate +1
$ = numOfDays