Converting Pandas DatetimeIndex to a numeric format

Convert to Timedelta and extract the total seconds from dt.total_seconds:

df

        date
0 2013-01-01
1 2013-01-02
2 2013-01-03
3 2013-01-04
4 2013-01-05
5 2013-01-06
6 2013-01-07
7 2013-01-08
8 2013-01-09
9 2013-01-10

pd.to_timedelta(df.date).dt.total_seconds()

0    1.356998e+09
1    1.357085e+09
2    1.357171e+09
3    1.357258e+09
4    1.357344e+09
5    1.357430e+09
6    1.357517e+09
7    1.357603e+09
8    1.357690e+09
9    1.357776e+09
Name: date, dtype: float64

Or, maybe, the data would be more useful presented as an int type:

pd.to_timedelta(df.date).dt.total_seconds().astype(int)

0    1356998400
1    1357084800
2    1357171200
3    1357257600
4    1357344000
5    1357430400
6    1357516800
7    1357603200
8    1357689600
9    1357776000
Name: date, dtype: int64