Hello, `to_excel` method with `engine='xlwt'` doesn't support `pandas.tslib.Timedelta` ``` import numpy as np import pandas as pd idx = pd.date_range('20140101', '20140201') df = pd.DataFrame(index=idx) df['col0'] = np.random.randn(len(idx)) s_idx = pd.Series(idx, index=idx) diff_idx = (s_idx-s_idx.shift(1)).fillna(pd.Timedelta(0)) df['diff_dt'] = diff_idx df.to_excel("out.xls", engine='xlwt') ``` it works fine with xlsx files (engine `openpyxl` or `xlsxwriter`). Kind regards