it is the date you apply for unemployment benefits that is essential in relation to the new rule from January 2019.
So the new rule will affect you regardless of you become member in an A-kasse in 2018 or later.
For a non-EU citizen you must have stayed in Denmark for at least 5 out of the last 12 years if you apply for benefits/get unemployed in 2019. If you apply in 2020 the requirement will be 6 out of the last 12 years, and from 2021 the requirement will be full implemented and be 7 out of the last 12 years.
As an non-EU citizen you can only include periods with stays in Denmark or other nordic countries during the last 12 years.
You can read more in our article about the new residence requirement.