How do I get the first datetime of the day?

Use the method localize()

to add a timezone:

from datetime import datetime
import pytz # $ pip install pytz

tz = pytz.timezone('America/Sao_Paulo')    
naive = datetime(2014, 10, 19)
aware = tz.localize(naive, is_dst=None)

      

        
        
        
      

    

If you run the code; it generates NonExistentTimeError

. How to deal with this error depends on the application, for example, to get some valid local time around midnight:

aware = tz.normalize(tz.localize(naive, is_dst=False))

      

        
        
        
      

    

Or you can increase the time by a minute until you get the actual local time (Sao Paulo):

from datetime import datetime, timedelta
import pytz # $ pip install pytz

tz = pytz.timezone('America/Sao_Paulo')
d = naive = datetime(2014, 10, 19)
while True:
    try:
        aware = tz.localize(d, is_dst=None)
    except pytz.AmbiguousTimeError:
        aware = tz.localize(d, is_dst=False)
        assert tz.localize(d, is_dst=False) > tz.localize(d, is_dst=True)
        break
    except pytz.NonExistentTimeError:
        d += timedelta(minutes=1) # try future time
        continue
    else:
        break

      

        
        
        
      

    

Result:

>>> aware
datetime.datetime(2014, 10, 19, 1, 0, tzinfo=<DstTzInfo 'America/Sao_Paulo' BRST-1 day, 22:00:00 DST>
>>> aware.strftime('%Y-%m-%d %H:%M:%S %Z%z')
'2014-10-19 01:00:00 BRST-0200'

      

        
        
        
      

    

Note: the first valid time 01:00

on that day. And the time zone is two hours below UTC (local = utc - 2).