GroupByBasedOnTimeInterval.cs

void Main()
{
	//my data entities
	var dataList = new List<DataModel>();
    for (int i = 0; i < 100; i++) {
        dataList.Add(new DataModel {
			SomeString = "some data",
            SomeValue = i,
            Timestamp = DateTimeOffset.UtcNow.AddMinutes(-(i+60))
        });
	}
	
	//my aggregation intervals, collected one hour apart
    var range = new List<TimeInterval>();
    var time = DateTimeOffset.UtcNow.RoundUpToHour().AddHours(+3);
	for (int i = 0; i < 100; i++) {
		range.Add(new TimeInterval {
					Start = time.AddHours(-(i+1)),
                    End = time.AddHours(-(i)),
                });
	}
	
	//dataList.Dump("original data");
	//range.Dump("timeframe range");
	//dataList.GroupBy(x => GetKey(range, x)).Select(x => new { x.Key, Val = x }).First().Dump("Gruped");
	dataList.GroupBy(x => GetKey(range, x)).ToDictionary(x => x.Key).Dump();
	
}


private static DateTimeOffset GetKey(List<TimeInterval> rg, dynamic item)
{
	//key range should cover the items always otherwise it will throw null ref exception
    var t = rg.FirstOrDefault(r => r.Start <= item.Timestamp && r.End >= item.Timestamp);
    return t.End;
}

public static class DateTimeOffsetHelper
    {
        public static DateTimeOffset RoundUpToHour(this DateTimeOffset dateTime)
        {
            DateTimeOffset previous = dateTime.RoundDownToHour();
            //if the given date time is already an exact hour, just return it
            //otherwise, add an hour to the rounded down previous hour and return that
            return dateTime == previous ? dateTime : previous.AddHours(1);
        }
        public static DateTimeOffset RoundDownToHour(this DateTimeOffset dateTime)
        {
            DateTimeOffset previous = new DateTimeOffset(dateTime.Year, dateTime.Month, dateTime.Day,
                                             dateTime.Hour, 0, 0, dateTime.Offset);
            return previous;
        }
    }
	
	 private class DataModel
        {
            public DateTimeOffset Timestamp { get; set; }
            public int SomeValue { get; set; }

            public string SomeString { get; set; }
        }

        private class TimeInterval
        {
            public DateTimeOffset Start { get; set; }
            public DateTimeOffset End { get; set; }
        }