Calculating number of weeks between 2 columns

Hi,

I have a calculated column which tells me the number of weeks a staff member worked for the company.

Length of Service = DATEDIFF(vw_CarersAll[StartDate],vw_CarersAll[DateLeft],WEEK)

What's tricky is that not all have a DateLeft, they are still with us.

I can cater for this by using 

Length of Service = DATEDIFF(vw_CarersAll[StartDate],TODAY(),WEEK)

My problem is how do I combine the two to give me total number of weeks each work has worked for the company?

Regards,

Gerry