What is a day, when does it start and end, etc, have various definitions around the world and in history. https://books.google.com/books/about/Calendrical_Calc

What is a day, when does it start and end, etc, have various definitions around the world and in history. https://books.google.com/books/about/Calendrical_Calculations_Millennium_Edit.html?id=5r_kSKE4OdYC https://books.google.com/books/about/Calendrical_Calculation... Calendrical Calculations is a fascinating intro the different calendar systems, along with computational methods of converting between systems.