import arcpy
from arcpy import env
env.workspace = 'the database'
butterfly = r'butterfly_2019' # the feature class

for i in butterfly['ID']:
    d1 = datetime.date.max('Datum')
    d2 = datetime.date.min('Datum')
    countdays = d2-d1
    butterfly['NrDays'] = antal

out_location = 'the database'
arcpy.FeatureClassToGeodatabase_conversion(butterfly , out_location)

import arcpy
from collections import defaultdict

fc = r'C:data.gdbsomefc'
datefield = 'DATE'
groupfield = 'GROUP'

#Create dictionary
d = defaultdict(list)
with arcpy.da.SearchCursor(fc,[groupfield,datefield]) as cursor:
    for group, date in cursor:
        d[group].append(date)
#d can now look like: defaultdict(<class 'list'>, {'OXFLYET': [datetime.datetime(2011, 4, 1, 0, 0), datetime.datetime(2011, 4, 1, 0, 0), datetime.datetime(2016, 11, 15, 0, 0), datetime.datetime(2011, 5, 1, 0, 0), datetime.datetime(2011, 5, 1, 0, 0)], 'MÖGSJÖN': [datetime.datetime(1992, 9, 1, 0, 0), None, None, datetime.datetime(1994, 1, 1, 0, 0)]})
#Each group is key (I only have two), and all dates are values in a list

#Calculate delta days per group
for group, datelist in d.items():
    datelist = [x for x in datelist if x is not None]
    if len(datelist) > 1:
        print('{0}: {1} days'.format(group, (max(datelist)-min(datelist)).days))
    else:
        print('{0}: not enough dates'.format(group))

OXFLYET: 2055 days
MÖGSJÖN: 487 days

df = pd.DataFrame.from_records(data=arcpy.da.SearchCursor(fc,[groupfield,datefield]), columns=[groupfield,datefield])
print(df.groupby(groupfield)[datefield].apply(lambda g: g.max() - g.min()))