removing time from date&time variable in pandas?

Assuming all your datetime strings are in a similar format then just convert them to datetime using to_datetime and then call the dt.date attribute to get just the date portion:

In [37]:

df = pd.DataFrame({'date':['2015-02-21 12:08:51']})
df
Out[37]:
                  date
0  2015-02-21 12:08:51
In [39]:

df['date'] = pd.to_datetime(df['date']).dt.date
df
Out[39]:
         date
0  2015-02-21

EDIT

If you just want to change the display and not the dtype then you can call dt.normalize:

In[10]:
df['date'] = pd.to_datetime(df['date']).dt.normalize()
df

Out[10]: 
        date
0 2015-02-21

You can see that the dtype remains as datetime:

In[11]:
df.dtypes

Out[11]: 
date    datetime64[ns]
dtype: object

You're calling datetime.datetime.strftime, which requires as its first argument a datetime.datetime instance, because it's an unbound method; but you're passing it a string instead of a datetime instance, whence the obvious error.

You can work purely at a string level if that's the result you want; with the data you give as an example, date_str.split()[0] for example would be exactly the 2015-02-21 string you appear to require.

Or, you can use datetime, but then you need to parse the string first, not format it -- hence, strptime, not strftime:

dt = datetime.strptime(date_str, '%Y-%m-%d %H:%M:%S')
date = dt.date()

if it's a datetime.date object you want (but if all you want is the string form of the date, such an approach might be "overkill":-).