VBS Field Default: Probation End Date calculation

Summary:

Can anyone help how can I mimic the logic of Probation End Date Calculation in VB Studio?
I need to create a defaulting rule to calculate the Probation End Date based on the Probation Period field values, and calculate them based on Effective Start Date. But instead of these fields, I need to create an Assignment DFF due to system restrictions on delivered fields (see reference Idea link below).

Example:

If Probation Period is 30 Days, calculate and default Probation End Date as Effective Start Date + 30

Reference:

Version (include the version you are using, if applicable): 25C

Code Snippet (add any code snippets that support your topic, if applicable): N/A