Hey Chris,

I have tried something along those lines, but in practice, I couldn't get the Date field to populate with the selected date properly.

I ended up doing something a little different, but similar to your idea. Instead of populating a dropdown, I used a date picker, and some javascript to only allow the selection of a proper date. I formatted the date picker to have the proper format, and that worked like a charm. Here's the snippet of javascript I ended up using (use the CSS class myDate on the datepicker):

$(document).ready(function(){
  function isCorrectDate(n) {
    var s = new Date("2017","03","08"); //Seed date, can be any appropriate date
    return (Math.ceil((s - n) / 86400000)%14==0); //Returns true on every day that is a two week increment from seed date
  }
  function checkDate(n) {
    return[isCorrectDate(n),""];
  }
  function checkField(input, inst) {
    if ($(input).closest('li').hasClass('myDate')) {
      $(input).datepicker("option", {beforeShowDay: checkDate});
    }
  }
  $.datepicker.setDefaults( {beforeShow: checkField} );
  window.Parsley.addValidator('secondsat', {
    validateString: function(value) {
      return isCorrectDate(new Date(value); //Validation condition, set to true to disable validation 
    },
    messages: {
      en: 'Not valid date.'
    }
  });
  $('.myDate input').attr('data-parsley-secondsat','');
})