In [1]: pd.to_datetime(["2012-01-01"] * 49 + ["2012-01-02 09"])
...
ValueError: unconverted data remains when parsing with format "%Y-%m-%d": " 09", at position 49. You might want to try:
    - passing `format` if your strings have a consistent format;
    - passing `format='ISO8601'` if your strings are all ISO8601 but not necessarily in exactly the same format;
    - passing `format='mixed'`, and the format will be inferred for each element individually. You might want to use `dayfirst` alongside this.

In [2]: pd.to_datetime(["2012-01-01"] * 50 + ["2012-01-02 09"])
...
ValueError: unconverted data remains when parsing with format "%Y-%m-%d": " 09", at position 1. You might want to try:
...