I thought I would forward this site to help in figuring out those elusive dates.
I had come across quite a few Ontario Death Certificates recently that had the person’s age in Years, Months and Days. I only had the year of their birth so I thought there must be a site that could calculate this for me.
Using Google I found this site: http://www.progenealogists.com/birthfromdeath.htm
Hope it will fill in some of those dates for you.