Sum with unique object

Hi @Badger , i see this topic Extract Month and Year from date field
So, I have the same trouble and i want a new field contain for example 2020-02
i try this code
grok { match => { "Date_of_entry" => "^%{YEAR:year}-%{MONTHNUM}" } }
but he extract just the month so what shoud i add to this code to get result like this 2020-02?
can you answer me please?