static void Main(string[] args)
        {
            var client = new MongoClient(new MongoClientSettings() { LinqProvider = Linq.LinqProvider.V3});
            var db = client.GetDatabase("d");
            var coll = db.GetCollection<Root>("c");

            coll.InsertOne(new Root() { ForSelectMany = new[] { new WitDateTimeOffset { Date = DateTimeOffset.UtcNow} } });

            var res = coll.AsQueryable()
                        .SelectMany(x => x.ForSelectMany)
                        .Select(y => new OutputWithMonth(y.Date.Month))
                        .ToList();
        }


        private class Root
        {
            public IEnumerable<WitDateTimeOffset> ForSelectMany { get; internal set; }
        }

        public class WitDateTimeOffset
        {
            public DateTimeOffset Date { get; set; }
        }

        private class OutputWithMonth
        {
            public int Month;

            public OutputWithMonth(int month) => Month = month;
        }