JSFiddle - React, Tailwind, and code Playground

HTML

<form action="advertResult.html">
    <label class="formText">First Name:</label>
    <input type="text" id="firstName" name="firstname" placeholder="" required>

    <label class="formText">Last Name:</label>
    <input type="text" id="lastName" name="lastname" placeholder="" required>

    <label for="commission" class="formText">Type of Commission</label>
    <select id="commission" name="commission" required>
      <option value="drawing">Art</option>
      <option value="video">Video</option>
    </select>

    <label for="doa" class="formText">Select a date for discussion</label>
    <input type="date" name="date" id="doa" max="2020-01-01" required>

    <input type="submit" value="Submit">
  </form>

JavaScript

document.querySelector('form').onsubmit = checkForm

function checkForm() {
  var answ = validateDate(document.getElementById("doa").value);
  if (answ == false) {
    return false;
  }
  return true;
}

function validateDate(doa) {
  var d = new Date(doa);
  var n = new Date();
  console.log(d, n)
  if (d <= n) {
    alert("Date cannot be before today");
    return false;
  }
  return true;
}