In the old days, determine length of service was simple. You just need to maintain a feature called ENTRY. This feature will determine to look at which date type on infotype 0041 or to use infotype 0000 action date as the hire date. A standard function module HR_ENTRY_DATE will read of this feature and life was happy.
Until recently when SAP introduce Management Global Employees (MGE) / Concurrent Employment (CE) functionality, all of that went down the tube. If you had installed the Best Practice Client w/ client 000 stuff, you should be safe with what is delivered. But with those who did not, you are into a big surprise.
You will need to maintain ENTRY feature as you normally would, but in addition need to maintain a whole new node of configuration in the IMG
IMG -> Personnel Management -> Personnel Administration -> Evaluation Basis -> Calculation of Employment Period
This entire nodes needs to be maintain. If you have a best practice client setup, the best practice setting will be enough for most implementation to work against. Enterprise Compensation Management module which uses senority setting in their guidelines will be able to leverage it as well.
Took me awhile to figure this out!
Have Fun w/ researching it.