The build date would be close to the date found at the top middle of your build sheet. If it is still visable it will be in Year/day format. The day will be listed in solar calendar/Ordinal date format, (often mistaken for Julian) meaning 00/60 would be the 60th day of 2000 or February 29th, 2000.
http://www.fourmilab.ch/documents/calendar/
http://www.fs.fed.us/raws/book/julian.shtml