dicts.SelectMany(d => d.Keys).Distinct().ToArray();

Dictionary<DateTime, double> dic = new Dictionary<DateTime, double>()
        {
            {DateTime.Now, 111}
        };

        Dictionary<DateTime, double> dic2 = new Dictionary<DateTime, double>()
        {
            {DateTime.Now, 111}
        };

        var list = dic.ToList();
        list.AddRange(dic2.ToList());

        var final = list.Distinct().ToDictionary(x => x.Key, x => x.Value);