So, I'd recommend trying first with a hardcoded value. Then if that works, switch to a Matillion variable. I tend to use the Text type rather than DateTime, since you have more control over the formatting that way.
Thanks @deBaris for your suggestion. Yes, <since> takes in only yyyy-MM-dd format.
However, I found that the <id> column is useful to determine new job runs incrementally using high water mark method. I was able to cast the <Id> field as 'INT' and load only the latest task history since the previous load.