How to truncate the time on a datetime object?

I think this is what you're looking for...

>>> import datetime
>>> dt = datetime.datetime.now()
>>> dt = dt.replace(hour=0, minute=0, second=0, microsecond=0) # Returns a copy
>>> dt
datetime.datetime(2011, 3, 29, 0, 0)

But if you really don't care about the time aspect of things, then you should really only be passing around date objects...

>>> d_truncated = datetime.date(dt.year, dt.month, dt.day)
>>> d_truncated
datetime.date(2011, 3, 29)

Use a date not a datetime if you dont care about the time.

>>> now = datetime.now()
>>> now.date()
datetime.date(2011, 3, 29)

You can update a datetime like this:

>>> now.replace(minute=0, hour=0, second=0, microsecond=0)
datetime.datetime(2011, 3, 29, 0, 0)

To get a midnight corresponding to a given datetime object, you could use datetime.combine() method:

>>> from datetime import datetime, time
>>> dt = datetime.utcnow()
>>> dt.date()
datetime.date(2015, 2, 3)
>>> datetime.combine(dt, time.min)
datetime.datetime(2015, 2, 3, 0, 0)

The advantage compared to the .replace() method is that datetime.combine()-based solution will continue to work even if datetime module introduces the nanoseconds support.

tzinfo can be preserved if necessary but the utc offset may be different at midnight e.g., due to a DST transition and therefore a naive solution (setting tzinfo time attribute) may fail. See How do I get the UTC time of “midnight” for a given timezone?

You cannot truncate a datetime object because it is immutable.

However, here is one way to construct a new datetime with 0 hour, minute, second, and microsecond fields, without throwing away the original date or tzinfo:

newdatetime = now.replace(hour=0, minute=0, second=0, microsecond=0)