Date Range Facet sort order

[Thomas Redman]

I am using the SolrJ Json facet API to retrieve a facet on a range of date.

            DateFacet df = (DateFacet)f;
            RangeFacetMap rfm = new RangeFacetMap(df.getField(), 
df.getStartDate(), df.getEndDate(),
                    df.getSpan().getKeyword())
                    .setMinCount(1);

I see no way to change the sort order to index (I want the output in date 
order), I get results sorted on the count. I have tried adding the sort field 
like so:

            rfm.put("sort","count asc");

but that had no effect. How does one change the sort order of the date range 
facets using the Json facet API?