I substituted the formula with their corresponding variable names and I get this error "Calculation contains a circular reference". 

Are you putting this in the input field for the Per_Diem_1 variable? If so, I think I misunderstood the intent from the original post. I had thought you would be taking the two date fields and the value of "Per Diem" to populate a completely different field.

I have a lookup set up that when a user enters a zip code, the per diem automatically gets populated with the daily rate. What I am trying to accomplish, is that when they select the depart/return dates, that it will multiply that per diem number that was already populated by the number of days. Is that why I am getting the calculation contains a circular reference? Do I need to create another field that will be just the per diem daily rate and then another one for the total which is where I would place the formula?

Thanks!

I added a "Per Diem Daily Rate" field. That one gets populated when inputting the zip code. On the "Per Diem" field, I used this formula "=DATEDIF(Date_Range_,Return_Date_ ,"d") * Per_Diem_Daily_Rate_"

It gives me that calculation error.

After some troubleshooting, I no longer get any calculation errors. The only thing now is that if I choose the same depart date and return date, it returns a value of 0. I want it to count as 1 day if the depart date and return date are the same. 

Would you still want "today" and "tomorrow" to be counted as 1 day in between?

If so you could try the formula

=IF(
  Date_Range_ = Return_Date_,
  Per_Diem_Daily_Rate_,
  DATEDIF(Date_Range_,Return_Date_,"d") * Per_Diem_Daily_Rate_
)

which should return the per diem rate (trivially multiplied by 1 day) if the dates are the same, and calculate the proper total otherwise.

EDIT: Updated to correct variable name

Today and tomorrow should count as 2 days. 

I added the formula to Javascript, however, it does not do anything. Notice how the "D" displays in red.

No, that's not JavaScript, you would add it in the Calculation field. However it sounds like that still isn't what you want.

What happens is each date (day, month, year) is mapped to a corresponding integer, similar to a UNIX timestamp. When calculating the DATEDIF value, these numbers are just subtracted to get the result. So if they are the same, n - n = 0. For the "today" and "tomorrow" case, (n + 1) - n = 1. etc. So we can just add 1 to this result to include both ends of the period, then multiply by the per diem rate:

=(DATEDIF(Date_Range_,Return_Date_,"d") + 1) * Per_Diem_Daily_Rate_

Is this better?

That works exactly how I wanted!

Thank you !