pandas to_datetime()

>>> import pandas as pd
>>> i = pd.date_range('20000101',periods=100)
>>> df = pd.DataFrame(dict(year = i.year, month = i.month, day = i.day))
>>> pd.to_datetime(df.year*10000 + df.month*100 + df.day, format='%Y%m%d')
0    2000-01-01
1    2000-01-02
...
98   2000-04-08
99   2000-04-09
Length: 100, dtype: datetime64[ns]

>>> df = df.astype(str)
>>> pd.to_datetime(df.day + df.month + df.year, format="%d%m%Y")
0    2000-01-01
1    2000-01-02
...
98   2000-04-08
99   2000-04-09
Length: 100, dtype: datetime64[ns]