If you want to solve it via Handlebars:
You can try if
`{{#checkIf (date createdAt ‘YYYYMM’) > 201810}} NEW {{/checkIf}}`
will work (it uses subexpression), but I suppose that it won’t as date helper will return string, not number.
In my opinion you will have to create a custom helper – in my opinion there is no other way to achieve this.
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