Complex Find sorting

I have some difficulties in doing the sorting in Find.

I have the following structure:

class TimeBlock
{
DateTime StartDate
}

class Event
{
ContentArea Times
}

The goal is to sort events by closest future (more than DateTime.Now + x hours) StartDate of their TimeBlocks.

I tried to index all StartDates like this:

[Ignore]
        public virtual DateTime[] StartDates
        {
            get
            {
                var dates = Times?.FilteredItems
                  ?.Select(x => x.GetContent())
                  .Where(b => b != null)
                  .OfType<EventInfoBaseBlock>()
                  .DefaultIfEmpty(new CourseInfoBlock { StartDate = DateTime.MinValue })
                  .Select(x => x.StartDate).ToArray()
                  ?? new[] { DateTime.MinValue };
                return dates;
            }
        }

and then do the ordering

Find.Search<Event>().Filter.....
.OrderBy(p => p.StartDates, p => p, p => p.GreaterThan(hideBeforeDate), SortMissing.Last, SortOrder.Ascending, SortMode.Min

but I get an exception “No mapping found for [StartDates.$$date] in order to sort on”

I'll be grateful for any help.