I have set the default template with time setting format as HH:MM:SS. When I create a new file it works fine. But when I download a spreadsheet file from my mail, sent by others, once again the time entries are changed to HH:MM:SS AM/PM.
why should your default setting overtake any settings from others???
I am not sure why? But it happens. Let me put it clearly. After setting a default template to HH:MM:SS, when I create a new sheet in Calc, when I enter 21:00:00 for time, it is fine. But when I download an Excel file(again made using Libre Office) made in a different computer, after I open and enter in one of the cells 21:00:00 for time, it changes to 9:00 PM. This is the problem that I am facing.
I understand what you want, but such a behavior isnt how Spreadsheets works.
You should think about another exchange-Format for your Data.