Podemos usar o operador de comparação
awk >$ cat ip.txt
Blah blah blah 2018-12-13
Blah blah blah 2018-02-13
Blah blah blah 2018-05-01
$ awk '$NF < "2018-05-01"' ip.txt
Blah blah blah 2018-02-13
Para obter linhas menores ou iguais a alguma data:
$ awk '$NF <= "2018-05-01"' ip.txt
Blah blah blah 2018-02-13
Blah blah blah 2018-05-01