138 (as there is always a non-numeric char in that string after a number we can omit the length of substr)
function a($)
$ = split($,'/')
$a = $[0]+'/'+$[1]+'/'+ (31-substr(_3_1_1__1_1,$[1]) + ($[0] mod 4=)*(($[0] mod 400=)+($[0] mod 100>))*(2=$[1]))
endfunction
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