If a dependant was active before the Plan start date, their effective date will be the first day of the Plan (i.e. had a kid in 2005 but the plan was active 2020-09-01, their effective date would 2020-09-01).
If the dependant was active after the first day of the plan (got married in Vegas to a stranger on 2020-09-02 but the plan was active 2020-09-01) their effective date would be 2020-09-02.
Common-law partners are eligible after 12 months of co-habitation. If your common-law status predates the plan start, their effective date will be 2020-09-01, if you hit the 12-month co-habitation marker after 2020-09-01, use the date you hit 12 months.