#!/usr/bin/env python3

# Change this with whatever function you want for grouping
def grouping(line):
    return int(line) // (24 * 3600 * 1000)

histo = {}
with open('some_file.csv', 'r') as file:
    for line in file.readlines():
        key = grouping(line)
        histo[key] = histo.get(key, 0) + 1

for k, v in histo.items():
    print('{},{}'.format(k,v))