var groupedFlights = departingFlights
    .OrderBy(f => f.DepartureDate)
    .Aggregate(
        new Dictionary<DateTime, List<Flight>>(),
        (acc, flight) =>
        {
            var key = acc.Keys.SingleOrDefault(k => IsDateWithin(flight.DepartureDate, k, k.AddMinutes(31)));
            if (key == default)
            {
                acc.Add(flight.DepartureDate, new List<Flight> { flight });
            }
            else
            {
                acc[key].Add(flight);
            }
            return acc;
        },
        groups => groups
            .Select(group => new FlightGrouping
            {
                GroupStart = group.Key,
                Flights    = group.Value
            }))
    .ToList();