For formatting a DataGridView Column that displays DateTime values, you set the DefaultCellStyle.Format property of the Column: see [^
] for examples.
Study the standard format specifiers for DateTime and TimeSpan here: [^
... and the Custom specifiers here: [^
You have still not answered the question about what you are using ObservableCollection for ... your code does not implement any handlers for its events.
Did you get serialize/desterilize working ?