I have a dataset that contains a date attribute and records show date/time. How can I count then number of unique dates while disregarding the time stamp?
If you can load your data into a pandas DataFrame (a "GeoAccessor" in the Python API), it's simple:
from arcgis.features import GeoAccessor
# load layer into dataframe
df = GeoAccessor.from_featureclass('path/to/fc')
# create new date field, dropping time
df['the_date'] = df['your_datetime_field'].dt.date
# get the count
df.groupby('the_date').count()