What can cause the date of submission status to change without a change of status "under review" for Elsevier editorial systems? I have a paper submitted to a journal of Elsevier.

From "Manuscript Submitted" to "with editor" took one day; then it was  "with editor" for five days, and then "under review". One and a half months later, the status of the paper was still "under review", but the date of status has changed.

What can cause the date in the submission system to change in the absence of a change in status?
When a paper is assigned to a reviewer, the reviewer may decline or fail to review the paper.  In this case the editor will assign the paper to a new reviewer.  The status date will change at the day the new review was assigned, but the status will remain the same.