Question

For example:

DateTime date1 = new DateTime(1955, 12, 12);
DateTime date2 = new DateTime(1967, 3, 6);
TimeSpan fff = date2 - date1;

Will it calculate number of days correctly? (taking leap year into account)

Answer 1

Yes, it takes this into account.

For proof, try:

DateTime date0 = new DateTime(2001, 12, 31);
DateTime date1 = new DateTime(2000, 12, 31);
DateTime date2 = new DateTime(1999, 12, 31);
Console.WriteLine("{0} / {1}", (date2 - date1).Days, (date1-date0).Days);

The above outputs: -366 / -365