Calculate duration and write to a text field

[Maria DePasquale]

I have two custom date fields (start and end).  I need to first calculate the duration (end-start) and then format that value like this:  d hh:mm:ss

Even if the duration is long, it would still be the above format so no months or years, just days hh:mm:ss.  If the duration was shorter than a day, the format would be hh:mm:ss.

I need to write the value to a text field.  So, the resulting text field would look something like this:  2  12:23:54  (2 days, 12 hours, etc)

I need to include this code in Post Function --> Scriptrunner.  Does anyone have a script handy that would help with this task?