Can we use the standard fields in a custom field formula?
Yes, it is possible to use the code of normal custom fields such as Start and Finish date, Due Date, Time Left, etc. for creating your own custom fields.
To find the formulas for these normal custom fields please do the following:
1. Go to: Tools/Custom Fields and click “Ins” button
2. In “Formula” field enter: $. The list of available formulas for standard custom fields will appear.
3. To use the code of the standard custom field in “Formula” field, double click the code in the list of standard custom fields