Sample file is attached to demonstrate the bug (?).
I am trying to summarize a range of fields within a repeating field with 31 repeats (month calcs).
The demo is now set for the whole range day 1 - 31
Press the button to add 1 to each field and see what happens when you reach field 10 and 14.
The calculation is done via a CustomFunction that takes the fieldname, startday and endday.
There may be a more elegant way to do this, but I don't see why the CF fails.