Please see definition from CDC NHSN below:

“Days present are the number of patients present at any time on a given day in each patient care location. Patient days are the number of patients present in each patient care location during the once daily census count.

For example, a patient admitted to the medical ward on Monday and discharged two days later, on Wednesday, will contribute three days present to the medical ward because the patient was there at some point during each of the three calendar days (Monday, Tuesday, and Wednesday). If this medical ward used midnight as the time of their daily census count, this same patient would only attribute two patient days because they were present for the midnight census count on Monday and Tuesday only.