Hope this helps: https://www.ircc.canada.ca/english/helpcentre/answer.asp?qnum=1543&top=14
What do I put for the date of birth (or death) of my family member if I don’t know it?
If you don’t know the complete date, or if it’s in the 1800s, enter 1900 in the space for the unknown year, and 01 for...