Numbers: conditonal highlight based on a date in a different column
This currently isn't possible. Conditional highlighting will only run the condition on the cell that the rule has been applied to.


  To change the appearance of a cell based on its cell value, create a conditional highlighting rule.


Source: Add conditional highlighting to cells — Numbers Help for Mac; emphasis mine.

The only exception to this is for conditions where a comparison is required — a second value can be referenced in the rule.


  Click  to use a cell reference. A cell reference lets you compare the cell’s value to another cell—so, for example, you can highlight a cell when its value is greater than another cell’s. Click a cell to select it, or enter its table address (for example, F1).


Source: Add conditional highlighting to cells — Numbers Help for Mac.

As a workaround, you could duplicate (or reference) the column A and place it next to column D, applying the conditional highlighting to that column instead. It won't be highlighting the cell with the data, but it'll give you something to glance at next to the data.