The Cypress 2.1.0 bundle calculates length of stay via the lengthOfStay method in hqmf_utils.js, subtracting the start datetime from the stop datetime (both stored in milliseconds), dividing by the number of milliseconds in 24 hours, and truncating the result to an integer, yielding the length of stay with granularity of days. This value is used when testing population criteria such as the "length of stay <= 120 day(s)" qualifier that occurs in the Initial Patient Population of many measures.

In all but one measure, the unit used in length of stay calculations is days. But CMS188v2 includes the following criterion:

"Occurrence A of Encounter, Performed: Encounter Inpatient (length of stay < 24 hour(s))"

Unfortunately it appears that Cypress ignores the units in this criterion and compares the calculated length of stay in days, as described above, with 24. Thus this criterion is matched for any length of stay less than 24 days rather than 24 hours as intended.

This same criterion exists (albeit with different nesting) in CMS188v3.