need clarification of a specific calculation

[ecowitness]

please refer to istsos2 / Tutorial / vp / FAO56.py

reference is to lines 81 thru 84 -- day of year == J

specifically line 83

why is the day of year reduced by one?

[massimiliano-cannata]

Reviewing the code I don't see any reason.. but has passed and cannot remember if there was a valid reason...

I would remove the -1 if no other motivation from others came up in a short period....