Use the named parameters:

say Date.new(year => 2019, month => 1);
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I don't see any other signature where the missing day would pass:

multi method new($year, $month, $day, :&formatter --> Date:D)
multi method new(:$year!, :$month = 1, :$day = 1  --> Date:D)
multi method new(Str $date                        --> Date:D)
multi method new(Instant:D $dt                    --> Date:D)
multi method new(DateTime:D $dt                   --> Date:D)
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