title: How to convert timeseries to %M:%S.%f starting with 00:00.00
body: <p>I have the output of a 20 min measurement with unregular frequency (the timedelta between each measuring point is different) stored in a csv file with unix timestamps and the time in iso format:</p>
  <pre><code>unix_timestamp      time_iso                        data  
  1602159331.000000   2020-10-08T12:15:31.00+00:00    10
  1602159331.152680   2020-10-08T12:15:31.15+00:00    3
  1602159331.222114   2020-10-08T12:15:31.22+00:00    5
  ...                 ...                             ...
  1602160747.73929    2020-10-08T12:35:31.00+00:00    7
  </code></pre>
  <p>The actual time is irrelevant. The only relevant time is the timedelta compared to the first measuring point. So actually I want this data:</p>
  <pre><code>time(%M:%S.%f)     data
  00:00.00           10
  00:00.15           3
  00:00.22           5
  ...                ...
  00:20.00           7
  </code></pre>
  <p>Is there a nice way to convert the time to the desired format but keep the information that it is a time related data type?</p>