I was single until 3 months into 2017. Now im common-law. When I claim eligible dependant it asks for a date. Do I enter the date when I becamr common-law?

Yes. This field should be prefilled from information entered in the setting up section of your return but if the field is blank, enter the date you became common-law.

Ensure that you meet the qualifications to claim the status of common-law. If your partner is not the other parent of your child, the rules for claiming this status are different.

https://turbotax.community.intuit.ca/replies/3465608