sim_df = pd.DataFrame({'delta_time_days': [1.02, 0.09, 1.08, 1.7, 4.1, 0.3,
    0.13, 0.01, 0.3, 0.7], 'cum_days': [1.1, 1.1, 2.2, 3.9, 8.0, 8.3, 8.4, 
    8.4, 8.8, 9.5]})
start = dt.datetime.now()
sim_df['date'] = start + pd.TimedeltaIndex(sim_df['cum_days'], unit='D')
Using datetime timedelta with a series in a pandas DF