Can a user defined field be used as length of service determinant in Derived Factor?

Summary:

There is a requirement to use the first ever date of working to determine the accrual rate. This would usually be prior to current employment. A flexfield segment on Assignment Attributes DFF has been used to hold this date. How can this be referenced in Derived Factors? I notice there is room for a formula. see below . If this is what this is used for, can someone provide an example formula on how to get the flex segment returned from the formula?

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