Where should I save?The file should NOT be saved in File.DirInternal or File.DirDefaultExternal as both these folders are deleted when the app is uninstalled
Like erel already statedWhere should I save?
A simple solution is to create a file somewhere on the external storage
This is correct. For a more robust solution you will need to use an online server which tracks the installation.The user in this case can always change the date of the saved file?
You can always encrypt you date string, keep it in ticks value, add a fixed string before encryption. .. there are many ways to at least make the user question him self "wff is this?"The user in this case can always change the date of the saved file?