Resolution:
Note that if the MigrationDateCalendar tool is used, and customer has some customerized control.html file, then after the upgrade of thier product of BWC or Formflow, the path of the image files will not be correctly upgraded.

The correct way to put the path of the image file is to insert the following part in the control.html file:

"<!--BeginIterator_EncodeUrl-->%%RenderRequest.Attribute('ContextPath')%
%<!--EndIterator_EncodeUrl-->/img/image cal.gif"

or

"<!--BeginIterator_EncodeUrl-->%%RenderRequest.Attribute('ContextPath')%
%<!--EndIterator_EncodeUrl-->/js/datetimepicker.js".

This %%RenderRequest.Attribute('ContextPath')%% is to get the right
Formflow context. By default it is "/Formflows" but they might modify it
at deployment time.

The <!--BeginIterator_EncodeUrl--> iterator is used to build the right
path in case the deployment is distributed. In that case /Formflows
won't work because it is on another Tomcat. This iterator build the
entire right path to reach the remote Tomcat.

If the customer has put the image file at somewhere else, then comparing paths in the page source code and the customer's deployed application, they can modify the path in the control.html arrocding to their actual location of the image files, and then rebuild their project .ear file and redeploy this new .ear file. Then it should work.

Date Calendar date picket not working after using MigrationDateCalendar tool