Note: this tutorial is meant for the premium version.
var value = data.value.split('/');
if(!value) return false;
if(value.length != 3) return false;
var d = new Date(value+'-'+value+'-'+value+'Z');
var dec1 = new Date('2018-01-01Z');
return d >= dec1;
Take note of the filter id ‘wof-validate-MMERGE3’. Replace MMERGE3 with the ID of your field. The ID can be found in your email list software such as MailChimp.