On a related note, here's an algorithm for parsing user input into a sanitized time format:

    String.prototype.toTime = function () {
      var time = this;
      var post_meridiem = false;
      var ante_meridiem = false;
      var hours = 0;
      var minutes = 0;

      if( time != null ) {
        post_meridiem = time.match( /p/i ) !== null;
        ante_meridiem = time.match( /a/i ) !== null;

        // Preserve 2400h time by changing leading zeros to 24.
        time = time.replace( /^00/, '24' );

        // Strip the string down to digits and convert to a number.
        time = parseInt( time.replace( /\D/g, '' ) );
      }
      else {
        time = 0;
      }

      if( time > 0 && time < 24 ) {
        // 1 through 23 become hours, no minutes.
        hours = time;
      }
      else if( time >= 100 && time <= 2359 ) {
        // 100 through 2359 become hours and two-digit minutes.
        hours = ~~(time / 100);
        minutes = time % 100;
      }
      else if( time >= 2400 ) {
        // After 2400, it's midnight again.
        minutes = (time % 100);
        post_meridiem = false;
      }

      if( hours == 12 && ante_meridiem === false ) {
        post_meridiem = true;
      }

      if( hours > 12 ) {
        post_meridiem = true;
        hours -= 12;
      }

      if( minutes > 59 ) {
        minutes = 59;
      }

      var result =
        (""+hours).padStart( 2, "0" ) + ":" + (""+minutes).padStart( 2, "0" ) +
        (post_meridiem ? "PM" : "AM");

      return result;
    };

I posted it a while back to the following thread:

https://stackoverflow.com/a/49185071/59087

One of my minor bug bears is why we humans have to enter time into input fields in such rigid formats when there's a straightforward algorithm to make such inputs far more natural.