No run_time timestamp in civicrm_job_log
Funny thing, while the Date column is filled out with a value in CiviCRM 4.6, it is empty in CiviCRM 4.7. I have checked on my customer's instances and the run_time column is in fact present but empty on 4.7. I will file a JIRA issue on this topic.

If your main concern is cleaning-up this table, you can easily do this by only keeping the last 1,000 or so entries in this table. This is easily done as the civicrm_job_log table, like many CiviCRM table, as an id column that is auto-increment. So you would do this to keep that last 1,000 entries:

-- from http://stackoverflow.com/questions/578867/sql-query-delete-all-records-from-the-table-except-latest-n/8303440#8303440
DELETE FROMcivicrm_job_log
  WHERE id &lt;= (
    SELECT id
    FROM (
      SELECT id
      FROMcivicrm_job_log
      ORDER BY id DESC
      LIMIT 1 OFFSET 1000 -- keep this many records
    ) foo
  );