Check if a date is "tomorrow" or "the day after tomorrow"

Question

I'm looking for a way to get "tomorrow" and "the day after tomorrow" returned from a function that receives to dates:

The current date:

"2015/04/24 18:15:00"

The date in the future:

"2015/04/25 02:40:00"

The function should return "tomorrow" here. I tried looking up some functions but they all return 0 not 1.

function days_between(date1, date2) {

    // The number of milliseconds in one day
    var ONE_DAY = 1000 * 60 * 60 * 24

    // Convert both dates to milliseconds
    var date1_ms = date1.getTime()
    var date2_ms = date2.getTime()

    // Calculate the difference in milliseconds
    var difference_ms = Math.abs(date1_ms - date2_ms)

    // Convert back to days and return
    return Math.round(difference_ms/ONE_DAY)

}

Any ideas?

Answer 1

First, here's the documentation on the Javascript Date object and its prototype.

So, how can you determine if a particular day (date2) is one day after another (date1)? Well, add one day to date1 and see if the dates match:

var date1_tomorrow = new Date(date1.getFullYear(), date1.getMonth(), date1.getDate() + 1);
if (date1_tomorrow.getFullYear() == date2.getFullYear() && date1_tomorrow.getMonth() == date2.getMonth() && date1_tomorrow.getDate() == date2.getDate()) {
    return "tomorrow"; // date2 is one day after date1.
}

If you want to determine if date2 is two days after date1, you can use the same logic as above but add 2 days instead of 1:

var date1_overmorrow = new Date(date1.getFullYear(), date1.getMonth(), date1.getDate() + 2);
if (date1_overmorrow.getFullYear() == date2.getFullYear() && date1_overmorrow.getMonth() == date2.getMonth() && date1_overmorrow.getDate() == date2.getDate()) {
    return "the day after tomorrow"; // date2 is two days after date1.
}

If the dates are not 1 or 2 days apart, the final bit would be to just return the date in your desired format:

return date2.toLocaleString(); // Firefox 29+, Chrome 24+, IE 11+
// OR
return date2.getFullYear() + '/' + (date2.getMonth() + 1) + '/' + date2.getDate() + ' ' + date2.getHours() + ':' + date2.getMinutes() + ':' + date2.getSeconds(); // Et al.

Test

function incrementDate(date, n) {
  return new Date(date.getFullYear(), date.getMonth(), date.getDate() + n);
}

function compareDates(date1, date2) {
  return date1.getFullYear() == date2.getFullYear() && date1.getMonth() == date2.getMonth() && date1.getDate() == date2.getDate();
}

function formatDate(date) {
  var year = date.getFullYear();
  var month = date.getMonth() + 1;
  var day = date.getDate();
  return year + '-' + (month < 10 ? '0' : '') + month + '-' + (day < 10 ? '0' : '') + day;
}

$(function() {
  var $output = $('#output');
  
  var comparisons = [
    {'start': new Date(2016, 6 - 1, 1), 'inc': 1, 'target': new Date(2016, 6 - 1, 2)},
    {'start': new Date(2016, 6 - 1, 30), 'inc': 1, 'target': new Date(2016, 7 - 1, 1)},
    {'start': new Date(2016, 12 - 1, 31), 'inc': 1, 'target': new Date(2017, 1 - 1, 1)}
  ];
  for (var i = 0, len = comparisons.length; i < len; i += 1) {
    var comp = comparisons[i];
    var $row = $('<tr>');
    
    $('<td>').text(formatDate(comp.start)).appendTo($row);
    $('<td>').text(comp.inc).appendTo($row);
    $('<td>').text(formatDate(comp.target)).appendTo($row);
    $('<td>').text(compareDates(comp.target, incrementDate(comp.start, comp.inc)) ? 'yes' : 'no').appendTo($row);
    
    $output.append($row);
  }
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<table>
  <thead>
    <tr>
      <th>Start</th>
      <th>Inc</th>
      <th>Target</th>
      <th>Equal</th>
    </tr>
  </thead>
  <tbody id="output">
  </tbody>
</table>

Answer 2

I would create two date objects. One that represents midnight of today's date and another that represents midnight of tomorrow. If the date I'm comparing is between those two dates then I know it falls under the umbrella of "tomorrow." If I want to do two days in advance the idea is the same but the date objects would represent an extra date in the future.

For example:

const oneDay = 1000 * 60 * 60 * 24;

function getMidnight(day){
  const date = new Date(day);
  date.setMilliseconds(999);
  date.setSeconds(59);
  date.setMinutes(59);
  date.setHours(23);
  return date;
}

function isTomorrow(date){
  const midnightTonight = getMidnight(new Date());
  const midnightTomorrow = new Date(midnightTonight.getTime() + oneDay);

  return date > midnightTonight && date < midnightTomorrow;
}

function isDayAfterTomorrow(date){
  const midnightTomorrow = getMidnight(new Date(Date.now() + oneDay));
  const midnightDayAfterTomorrow = new Date(midnightTomorrow + oneDay);

  return date > midnightTomorrow && date < midnightDayAfterTomorrow;
}