The Last Combined Date pulls the latest Close Date of all donations that qualify as either a hard or soft credit.  Since this is a Giving Summaries field, it updates with the overnight task that updates Giving Summaries.  IF the latest Close Date of all donations that qualify is modified to an earlier or later date, the Last Combined Date will update accordingly.

Note: The latest Close Date can be later than the current date, which will result in the Last Combined Date being later than the current date.