When you request facet information for a uint field, ASN Search returns the min and max values for the field. For example, when you specify facet=date_valid, you get the first and last year for active, published standards that appears in the date_valid field:
"facets":{"year":{"min":1993,"max":2012}}
To drill down into particular bins of integers, you use the facet-FIELD-constraints parameter.
For more information, see Getting Facet Information for Particular Values.