In the promptset, a prompt of type "date" has a default value of &$PHYS_DATE_YYYYMMDD#
When this promptset is used in a job, and you execute the job, the prompt's variable is empty.
If you change the value of the prompt during execution, then the prompt's variable has the selected value.
The allowed date format of a date field with a predefined variable is yyyy-mm-dd (this info is currently undocumented).
Use the predefined variable &$PHYS_DATE_YYYY_MM_DD_d#