Formula edit for finding out a date when criteria are met

I have three columns, one is start date (P3), one is end date(Q3), and one is total time (R3). I have the time one set right now with

=DATEDIF(P3,TODAY(),"y")&"y "&DATEDIF(P3,TODAY(),"ym")&"m "&DATEDIF(P3,TODAY(),"md")&"d"

so it will show a running total of how long it has been from start to today. I want to have it so once you enter a date in the end date column it uses that as the end date instead of calculating to today.

Can someone help me with that?