How to format hh:mm:ss for time periods greater than 24 hours

[anotherrachel]

Hello
I have a set of data (approx. 200,000 rows) and a calculation to determine the time between each row. The difference between each row is between 30 seconds to 5 minutes so I have formatted this data as hh:mm:ss
When I sum the total data set

(or various subtotals) ​

the total time taken is greater than 24 hours so the format hh:mm:ss is not great enough to show the full time taken.
I tried to apply the format dd:hh:mm:ss but it did not seem to work.
I appreciate any ideas on the format I should be using.
Thank you!

 

[RoryA]

Use [hh] in place of hh to go over the 24 hours.

 

[anotherrachel]

Thanks very much!