The Scrum 30 days or less is Calendar 30 days, or Net 30 days
For the purpose of development and QA planning, we are setting only 20 net days for the maximum length for The Scrum, because the calendar 30 days include weekends and holidays in different countries. 30 days minus 9 days (average 4 to 5 Saturdays and Sundays) minus 1 day (average national holiday in one month) = 20 days. By case by case, we reduce it to 15 days.
Any project plan exclude the holidays in addition to the weekends as non-working days. So, I understand that the Scrum 30 days maximum duration rule means "Maximum Net 20 days".
Is my understanding correct? Please let me know of your thoughts.
The Scrum Guide says that Sprints "are fixed length events of one month or less". A more sophisticated interpretation is unlikely to prove helpful.
I've never needed anything better than "one month or less". As @Ian Mitchell said, I don't see where trying to get as exact as you are is really worth doing.
In addition to what the others mentioned, complex product development comes with a lot of uncertainty and many unknowns. Most likely things will change during the Sprint and new learnings will emerge. As the saying goes, plans are worthless, planning is everything. Those of us who have worked with Scrum long enough have become comfortable with uncertainty, and see Developers replanning every day for the next 24 hours.
By the way planning a Sprint for a month, compared to shorter Sprints, isn't forbidden but does adds an extra challenge.
All the best!
Thanks for all of your comments. As conclusion, the Scrum doesn't specify any more than 30 days maximum, neither it includes or excludes non-working days. However, it doesn't improve the efficiency to be so precise, but the effort shall be spent for day-to-day planning stage as complexity and uncertainty are the big variables. Certainly, it is just a framework. I can leave as is how we are operating now.