I'm sorry your old girl is having problems. You will know when the time comes. Your dog will let you know. It is never easy. Tell your coworker what you are facing so she will be ready to take over for you for a day. My heart to yours. It is never easy to say good bye but leaving them have some dignity in the end is better. When the day comes and I hope she does make it to 13 then spend time with her, talk to her, tell her that a time is coming when she won't be in pain. I remember my last night with Pyrate. I knew the time had come as he had a stroke or seizure at home and it left him not cognizant of any one around him but he knew I was there. I held him all night knowing it would be our last. It still hurts after two years.