How to create rows based on dates on seperate rows?

It requires some coding (steps Custom1 and Custom2 below).

Transform the dates to numbers, and make a list from the minimum value to the maximum value.

Turn this into a table.

Now you can use standard menu-options:

Transform numbers back to dates.

Merge with the original table and fill down the missing values.

let
    Source = Table1,
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", Int64.Type}}),
    Custom1 = {List.Min(#"Changed Type"[Date])..List.Max(#"Changed Type"[Date])},
    Custom2 = Table.FromColumns({Custom1},{"date"}),
    #"Changed Type1" = Table.TransformColumnTypes(Custom2,{{"date", type date}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type1",{"date"},Table1,{"Date"},"Table1",JoinKind.LeftOuter),
    #"Expanded Table1" = Table.ExpandTableColumn(#"Merged Queries", "Table1", {"user", "Key"}, {"user", "Key"}),
    #"Filled Down" = Table.FillDown(#"Expanded Table1",{"user", "Key"})
in
    #"Filled Down"