I have a csv-file I read in with pandas to a dataframe. The time column is formatted in days since 01-01-1900. The aim is to gain an isoformatted column with the "T" between date and time. The csv-file looks like: DateTime,column1,column2,column3,column4,column5 43621.6210327662,1.234,2.665,0.888,3.891,3.762 43621.6210445023,2.232,1.233,0.888,3.789,3.524 My code to read in is: import pandas as pd df = pd.read_csv(myfile.csv) df and the dataframe contains days since 01-01-1900: DateTime column1 column2 column3 column4 column5 0 43621.621033 1.234 2.665 0.888 3.891 3.762 1 43621.621045 2.232 1.233 0.888 3.789 3.524 Now I convert the days-formatted date to a more common look: from datetime import datetime, timedelta df['DateTime'] = pd.to_datetime('1900-01-01') + pd.to_timedelta(df['DateTime'],'D') df DateTime column1 column2 column3 column4 column5 0 2019-06-07 14:54:17.230999464 1.234 2.665 0.888 3.891 3.762 1 2019-06-07 14:54:18.244998936 2.232 1.233 0.888 3.789 3.524 My aim is to have a isoformatted DateTime column that has the "T" between date and time like: DateTime column1 column2 column3 column4 column5 0 2019-06-07T14:54:17.230999464 1.234 2.665 0.888 3.891 3.762 1 2019-06-07T14:54:18.244998936 2.232 1.233 0.888 3.789 3.524 But the DateTime column seems to be a series instead of a date? I can`t apply something like: df['DateTime'].isoformat() How would I proceed? Continue reading...