Cumulative tenure year up to the selected month end date

[Newbie777]

Hello,

I am trying to calculate cumulative tenure years for employee employment length.  I have created employee headcount chart by month end in multiple years, where I can click selected month end and get the list of employee names and the number of employees at any time point.  I am now trying to add the information of the cumulative tenure which will be reduced accordingly when past dates are selected.  I'd appreciate it if you could let me know how I can modify the measure expression below to show the cumulative tenure figures up to the selected point in time instead of the tenure figure which gets filtered and chopped by the selected month end?  

Thank you & best regards,