df.loc[:,'est_arr_date'] = df.loc[:,'estimated_arrival_date'].apply(convert_StrTime)
df.loc[:,'act_arr_date'] = df.loc[:,'actual_arrival_date'].apply(convert_StrTime)
df['delta'] = df['est_arr_date'] - df['act_arr_date']

A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead