Seniority date field how to have it automatically calculated for rehired workers?

Hello,

For the field seniority date in fusion , we would like to have it calculated automatically especially when an employee leaves and is terminated, and then is rehired after more than one month. Currently , it looks like this field isn’t calculated in the system. 

How can we have the seniority date field automatically calculated ? (terminated workers that are rehired after awhile)

See attached file with the example.  field under employment

Thanks a lot

Revital.