title: How to write/read time stamp from a variable in airflow?
body: <p>I'm working with:  <code>EXEC_DATE = '{{ macros.ds_add(ds, 1) }}'</code> This gives me the execution date but not the hour.
  I want to be able to save this value as <code>YYYY-MM-DD HH:MM</code> into variable called <code>process_last_run</code>.</p>
  
  <p>Basically read the variable in the beginning of the run and write to it in the end of the dag. This variable indicates what was the running time of the last dag.</p>
  
  <p>How can I do that?</p>