Difficulty getting difference between two times

Hello everyone,
I'm having difficulty getting the difference between two time fields in Minutes. Both are strings {Arrival_time} and {Appointment_time}. They are both strings displaying time as 00:00 AM/PM. Also, {Arrival_time} is sometimes blank.

I used this formula to replace the "" as N/A:
if IsNull({appt_data.time_of_arrival}) then
"<null>"
else
if {appt_data.time_of_arrival} = "" then
    "N/A"
else
    CStr (Time({appt_data.time_of_arrival}), "h:mm:ss tt")

If I filter out the "N/A" and use the formula:
datediff("n",datetime({@Time arrival time}),datetime({appt_data.appointment_start_time}))

It works. If I bring back the "N/A" I get the "Bad Time Format String" message.

Please help!
Thanks

Edited by Andrewarbogast - 09 Oct 2017 at 7:09am

what do you want to do with the empty strings?
return a 0?
return a "N/A" string?

I was able to figure it out so that it returns a zero but the problem is if I wanted to do an average then the 0 would skew the data. So maybe it needs to return a a "N/A"

that wont help your issue with the average.
use a running total as an average with a evaluate formula to exclude rows where IsNull({appt_data.time_of_arrival})