<input type="date">

<input type="date" value="2017-06-01" />

var dateControl = document.querySelector('input[type="date"]');
dateControl.value = "2017-06-01";
console.log(dateControl.value); // prints "2017-06-01"
console.log(dateControl.valueAsNumber); // prints 1496275200000, a UNIX timestamp

<form action="https://example.com">
  <label>
    Enter your birthday:
    <input type="date" name="bday" />
  </label>

  <p><button>Submit</button></p>
</form>

<form>
  <label for="party"
    >Choose your preferred party date:
    <input type="date" name="party" min="2017-04-01" max="2017-04-30" />
  </label>
</form>

<form>
  <label>
    Choose your preferred party date (required, April 1st to 20th):
    <input
      type="date"
      name="party"
      min="2017-04-01"
      max="2017-04-20"
      required />
    <span class="validity"></span>
  </label>

  <p>
    <button>Submit</button>
  </p>
</form>

label {
  display: flex;
  align-items: center;
}

span::after {
  padding-left: 5px;
}

input:invalid + span::after {
  content: "✖";
}

input:valid + span::after {
  content: "✓";
}

<form>
  <label for="bday"
    >Enter your birthday:
    <input type="date" name="bday" required pattern="\d{4}-\d{2}-\d{2}" />
    <span class="validity"></span>
  </label>
  <p>
    <button>Submit</button>
  </p>
</form>

span {
  position: relative;
}

span::after {
  right: -18px;
  position: absolute;
}

input:invalid + span::after {
  content: "✖";
}

input:valid + span::after {
  content: "✓";
}

<form>
  <label class="nativeDatePicker"
    >Enter your birthday:
    <input type="date" name="bday" />
    <span class="validity"></span>
  </label>

  <fieldset class="fallbackDatePicker" hidden>
    <legend class="fallbackLabel">Enter your birthday:</legend>

    <label>
      Day:
      <select name="day"></select>
    </label>

    <label>
      Month:
      <select name="month">
        <option>January</option>
        <option>February</option>
        <option>March</option>
        <option>April</option>
        <option>May</option>
        <option>June</option>
        <option>July</option>
        <option>August</option>
        <option>September</option>
        <option>October</option>
        <option>November</option>
        <option>December</option>
      </select>
    </label>

    <label>
      Year:
      <select name="year"></select>
    </label>
  </fieldset>
</form>

span {
  padding-left: 5px;
}

input:invalid + span::after {
  content: "✖";
}

input:valid + span::after {
  content: "✓";
}

// define variables
var nativePicker = document.querySelector(".nativeDatePicker");
var fallbackPicker = document.querySelector(".fallbackDatePicker");

var yearSelect = document.querySelector("[name=year]");
var monthSelect = document.querySelector("[name=month]");

// Test whether a new date input falls back to a text input
var testElement = document.createElement("input");

try {
  test.type = "date";
} catch (e) {
  console.log(e.description);
}

// If it does, run the code inside the if () {} block
if (testElement.type === "text") {
  // hide the native picker and show the fallback
  nativePicker.hidden = true;
  fallbackPicker.hidden = false;

  // populate the days and years dynamically
  // (the months are always the same, therefore hardcoded)
  populateDays(monthSelect.value);
  populateYears();
}

function populateDays(month) {
  const daySelect = document.querySelector("[name=day]");
  const month = monthSelect.value;

  // Create variable to hold new number of days to inject
  let dayNum;

  // 31 or 30 days?
  switch (month) {
    case "April":
    case "June":
    case "September":
    case "November":
      dayNum = 30;
      break;
    case "February":
      // If month is February, calculate whether it is a leap year or not
      const year = yearSelect.value;
      const isLeap = new Date(year, 1, 29).getMonth() === 1;
      dayNum = isLeap ? 29 : 28;
      break;
    default:
      dayNum = 31;
  }

  // inject the right number of new <option>s into the <select>
  daySelect.options = Array.from({ length: dayNum }, function (index) {
    return index + 1;
  });

  // if previous day has already been set, set daySelect's value
  // to that day, to avoid the day jumping back to 1 when you
  // change the year
  if (previousDay) {
    daySelect.value = previousDay;

    // If the previous day was set to a high number, say 31, and then
    // you chose a month with fewer days in it (like February),
    // this code ensures that the highest day available
    // is selected, rather than showing a blank daySelect
    if (previousDay > daySelect.length + 1) {
      daySelect.selectedIndex = daySelect.length;
    }
  }
}

function populateYears() {
  // get this year as a number
  var year = new Date().getFullYear();

  // Make this year and the 100 years before it available in the <select>
  daySelect.options = Array.from({ length: 100 }, function (index) {
    return index + year;
  });
}

// when the month or year <select> values are changed, rerun populateDays()
// in case the change affected the number of available days
yearSelect.onchange = populateDays;
monthSelect.onchange = populateDays;

// preserve day selection
var previousDay;

// update what day has been set to previously
// see end of populateDays() for usage
daySelect.onchange = function () {
  previousDay = daySelect.value;
};