Nor sure to really understand your requirement, but what about this: 
> for <1 5 3 13> -> $month { say Date.new(2019, $month, 1 )}
2019-01-01
2019-05-01
2019-03-01
Month out of range. Is: 13, should be in 1..12
  in block <unit> at <unknown file> line 1

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In reply to Re^3: strptime("%Y-%m") in perl6 by Laurent_R
in thread strptime("%Y-%m") in perl6 by leszekdubiel

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