New
#11
Not at my personal desktop but it would be the date I built the system as it would be on every computer I've ever made for myself. I've never had to reformat and reinstall windows on any machine for any reason as long as it was an actively used machine which can last anywhere from 3-5 years or so. Then they move over to being a secondary/guest/experimental machine for another few years.
My secondary machines get all kinds of experimental things installed on them and don't really count (i.e. curently is my win8 test machine), but my main machines have never needed to be reformatted "just because"