A date range selector lets users pick a time frame between a start and end date, which is useful in booking trips, sorting info by date blocks, picking time slots, and planning schedules.

I’m going to show you an example where, even though JavaScript is involved, the bulk of the work is handled by the “n of selector(s)” syntax of the CSS :nth-child selector, making it easy to build the range selection.

The “n of selector” syntax

This syntax of the :nth-child selector filters elements by a given selector first among all the child elements, before selecting them by a counting order.

<p>The reclamation of land...</p>
<p>The first reclamations can be traced...</p>
<p class="accent">By 1996, a total of...</p>
<p>Much reclamation has taken...</p>
<p class="accent">Hong Kong legislators...</p>

.accent {
  color: red;
}
.accent:nth-child(2) {
  font-weight: bold; /* does not work */
}
:nth-child(2 of .accent){
  text-decoration: underline;
}

There are two .accent-ed paragraphs with red text. As we try to target the second accented paragraph, .accent:nth-child(2) fails to select it because it’s trying to find an .accent element that’s the second child of its parent.

Whereas, :nth-child(2 of .accent) succeeds in selecting and styling the second accented paragraph because it’s only looking for the second element among the **.accent** elements rather than the second of all of the children.

The Layout

Moving onto our main example, let’s put together a month layout. It only takes a few lines of CSS.

<ul id="calendar">
  <li class="day">Mon</li>
  <li class="day">Tue</li>
  <!-- up to Sat -->
  <li class="date">01<input type="checkbox" value="01"></li>
  <li class="date">02<input type="checkbox" value="02"></li>
  <!-- up to 31  -->
</ul>

#calendar {
  display: grid;
  grid-template-columns: repeat(7, 1fr); /* 7 for no. of days in a week */
}

Choose Only Two Dates

Now is when we reach for JavaScript since we can’t check/uncheck a control in CSS. But even here the “n of selector” syntax can be very useful.

When we pick two dates to create a range, clicking on a third date will update the range and remove one of the earlier dates.

You can set up the range re-adjustment logic in any way you like. I’m using this approach: If the third date is either earlier or later than the last return date, it becomes the new return date, and the old one is unselected. If the third date is earlier than the last onward date, it becomes the new onward date, and the old one is unselected.

const CAL = document.getElementById('calendar');
const DT = Array.from(CAL.getElementsByClassName('date')); 

CAL.addEventListener('change', e => {
  if (!CAL.querySelector(':checked')) return;
  
  /* When there are two checked boxes, calendar gets 'isRangeSelected' class  */
  CAL.className = CAL.querySelector(':nth-child(2 of :has(:checked))') ? 'isRangeSelected':'';

  /* When there are three checked boxes */
  if (CAL.querySelector(':nth-child(3 of :has(:checked))')) {

    switch (DT.indexOf(e.target.parentElement)) {

      /* If the newly checked date is first among the checked ones, 
          the second checked is unchecked. Onward date moved earlier. */
      case DT.indexOf(CAL.querySelector(':nth-child(1 of :has(:checked))')):
      CAL.querySelector(':nth-child(2 of :has(:checked)) input').checked = 0; 
      break;

      /* If the newly checked date is second among the checked ones, 
          the third checked is unchecked. Return date moved earlier. */
      case DT.indexOf(CAL.querySelector(':nth-child(2 of :has(:checked))')):
      CAL.querySelector(':nth-child(3 of :has(:checked)) input').checked = 0; 
      break;

      /* If the newly checked date is third among the checked ones, 
          the second checked is unchecked. Return date moved later. */
      case DT.indexOf(CAL.querySelector(':nth-child(3 of :has(:checked))')):
      CAL.querySelector(':nth-child(2 of :has(:checked)) input').checked = 0; 
      break;

    }
  }
});

First, we get the index of the current checked date (DT.indexOf(e.target.parentElement)), then we see if that’s the same as the first checked among all the checked ones (:nth-child(1 of :has(:checked))), second (:nth-child(2 of :has(:checked))), or third (:nth-child(3 of :has(:checked))). Given that, we then uncheck the relevant box to revise the date range.

You’ll notice that by using the “n of selector” syntax, targeting the :checked box we want by its position among all checked ones is made much simpler — instead of indexing through a list of checked dates in JavaScript for this, we can directly select it.

Styling the range is even easier than this.

Styling the Range

/* When two dates are selected */
.isRangeSelected { 
  /* Dates following the first but not the second of selected */
  :nth-child(1 of :has(:checked)) ~ :not(:nth-child(2 of :has(:checked)) ~ .date) {
    /* Range color */
    background-color: rgb(228 239 253); 
  }
}

When there are two dates chosen, the dates between the first (1 of :has(:checked)) and second (2 of :has(:checked)) are colored pale blue, creating a visual range for that block of dates in the month.

The color is declared inside a compound selector that selects dates (.date) following the first of all checked date (:nth-child(1 of :has(:checked))), but not the second of all checked date (:not(:nth-child(2 of :has(:checked))).

Here’s the full example once again: