In an attempt to answer this question, I discovered that the Post Form on Value change option is missing on that component. I've logged an incident in the bug tracking system to get it fixed (pending review/approval as a defect) I also noticed the Calender component is missing the same option (another incident). The time picker component was also missing the option in current code, but that's already been identified and fixed in current dev builds.
The Date picker component does have this option, so if you can ignore the time portion, that's the solution with the least UI confusion.
I next reviewed using Custom Events, but the DateTime Picker doesn't appear to fire them, or at least it doesn't contain onChange, and onBlur doesn't appear to fire (logged another bug).
For your immediate usage, the next best alternative would be to use the Refresh Button. That will trigger any embedded dynamic models to recalculate. Be sure to declare (Initialize Data component) your DateTime variables before entering the form component, that way your logic can be consistent regardless of first time entry or a revisit to the same page.