也许帮助剥离最后6个字符：
print df
datetime
0 2015-12-01 00:00:00-06:00
1 2015-12-01 00:00:00-06:00
2 2015-12-01 00:00:00-06:00
df['datetime'] = df['datetime'].astype(str).str[:-6]
print df
datetime
0 2015-12-01 00:00:00
1 2015-12-01 00:00:00
2 2015-12-01 00:00:00
602392714
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