public class Item
{
    private DateTime _checkIn;
    private DateTime _checkOut;

    public Item(DateTime checkIn, DateTime checkOut) {
        _checkIn = checkIn;
        _checkOut = checkOut;
    }

    public DateTime GetCheckInDate() => _checkIn;
    public DateTime GetCheckOutDate() => _checkOut;
    public void SetCheckInDate(DateTime value) => _checkIn = value;
    public void SetCheckOutDate(DateTime value) => _checkOut = value;

}

void Main()
{
    var items = new[] {
        new Item(DateTime.Parse("2017-05-01"), DateTime.Parse("2017-05-01")),
        new Item(DateTime.Parse("2017-05-01"), DateTime.Parse("2017-05-10")),
        new Item(DateTime.Parse("2017-05-03"), DateTime.Parse("2017-05-03")),
        new Item(DateTime.Parse("2017-05-10"), DateTime.Parse("2017-05-14")),
        new Item(DateTime.Parse("2017-05-12"), DateTime.Parse("2017-05-12")),
        new Item(DateTime.Parse("2017-05-14"), DateTime.Parse("2017-05-19")),
        new Item(DateTime.Parse("2017-05-19"), DateTime.Parse("2017-05-22"))
    };


    var offset = items[0].GetCheckInDate().Date - DateTime.Parse("2017-01-01");

    foreach (var item in items)
    {
        item.SetCheckInDate(item.GetCheckInDate() - offset);
        item.SetCheckOutDate(item.GetCheckOutDate() - offset);
    }

    items.Select(_ => $" {_.GetCheckInDate():yyyy-MM-dd} {_.GetCheckOutDate(): yyyy-MM-dd}")
        .ToList().ForEach(Console.WriteLine);
}