The default value of a date parameter must be an integer (BMXAA8268W)

Problem(Abstract)

When setting a "Default value" for a date parameter you cannot specify a specific date. You must specify a number. In previous versions, a specific date could be used as the default, this no longer applies as of Maximo 7.5

Cause

The functionality was changed by design.

Resolving the problem

For date parameters the purpose of "Default value" is to set an offset from the current day. For example, if you have the number 1 as the Default value, your date parameter will display on the request page with a default of tomorrow. The 1 tells it to show tomorrow. Likewise 2 would be 2 days from today and 7 would be 1 week from today. To use a date prior to today, use a negative number -1 would be yesterday, etc. This allows relative dates to appear as default values. 

Depending on your requirement, the new functionality may, or may not apply to your situation but it is not a bug and would be considered as an enhancement request if you wish to utilize the previous behavior.