Using Java Stream to count occurrences of Dates in a list of items

To count you're looking for something like:

Map<Instant, Long> foundInstants =  foundDates.stream()
            .collect(Collectors.groupingBy(Date::toInstant, Collectors.counting()));

to add to that you could cut short those if..else into :

ExtendedDataSeriesItem seriesItem = 
        new ExtendedDataSeriesItem(c.toInstant(), foundInstants.getOrDefault(c.toInstant(), 0L));
seriesItem.setSeriesType("singleDataPoint");
series.add(seriesItem);

and this goes by saying that you should at the same time look for migrating to LocalDateTime and refrain from using Date.

You can use groupingBy() and then use the downstream collector counting().

Map<Date, Long> occurrances = dateList.stream().collect(
                  groupingBy(d -> yourTransformation(d), counting()));

It should be easy enough to create your DataSeriesItem objects from that map.