What's the most elegant way to get the end of the day (datetime)?

Using dict.get can simplify your code. It is a bit cleaner than using datetime.replace and timedelta objects.

Here's something to get you started:

from datetime import datetime

options = dict(month=5, day=20)
now = datetime.now()
start_time = datetime(year=options.get('year', now.year), 
                      month=options.get('month', 1),
                      day=options.get('day', 1)
                      hour=0,
                      minute=0,
                      second=0)
stop_time =  datetime(year=options.get('year', now.year), 
                      month=options.get('month', now.month),
                      day=options.get('day', now.day),
                      hour=now.hour,
                      minute=now.minute,
                      second=now.second)

To set the stop_time, advance start_time one year, month or day as appropriate, then subtract one timedelta(microseconds=1)

if options['year']:
    start_time = start_time.replace(year=options['year'], month=1, day=1)
    stop_time = stop_time.replace(year=options['year']+1)-timedelta(microseconds=1)

elif options['month']:
    start_time = start_time.replace(month=options['month'], day=1)
    months=options['month']%12+1
    stop_time = stop_time.replace(month=months,day=1)-timedelta(microseconds=1)

else:
    start_time = start_time.replace(day=options['day'])
    stop_time = stop_time.replace(day=options['day'])+timedelta(days=1,microseconds=-1)

    today = datetime.date.today()
    begintime = today.strftime("%Y-%m-%d 00:00:00")
    endtime = today.strftime("%Y-%m-%d 23:59:59")