JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>JS Bin</title>
</head>
<body>
  
    Output format: <input type="text" id="format" value="g:ia / H:i" />
    <br>
    <br><br>
    Step: <select id="step">
    <option value="1">1</option>
    <option value="5">5</option>
    <option value="10">10</option>
    <option value="15">15</option>
    <option value="30">30</option>
    <option value="60">60</option>
    </select>
    <hr>
  Try entering a variety of date formats:<br><br>
  Time: <input type="text" id="timepicker" size="8" placeholder="12:00am" /> &rarr; <span id="result"></span>
  
</body>
</html>

CSS

BODY {
    font: 14px sans-serif;            
    padding: 40px;
}

HR {
    margin: 20px 0;
}

JavaScript

function parseTime(time, format, step) {
	
	var hour, minute, stepMinute,
		defaultFormat = 'g:ia',
		pm = time.match(/p/i) !== null,
		num = time.replace(/[^0-9]/g, '');
	
	// Parse for hour and minute
	switch(num.length) {
		case 4:
			hour = parseInt(num[0] + num[1], 10);
			minute = parseInt(num[2] + num[3], 10);
			break;
		case 3:
			hour = parseInt(num[0], 10);
			minute = parseInt(num[1] + num[2], 10);
			break;
		case 2:
		case 1:
			hour = parseInt(num[0] + (num[1] || ''), 10);
			minute = 0;
			break;
		default:
			return '';
	}
	
	// Make sure hour is in 24 hour format
	if( pm === true && hour > 0 && hour < 12 ) hour += 12;
	
	// Force pm for hours between 13:00 and 23:00
	if( hour >= 13 && hour <= 23 ) pm = true;
	
	// Handle step
	if( step ) {
		// Step to the nearest hour requires 60, not 0
		if( step === 0 ) step = 60;
		// Round to nearest step
		stepMinute = (Math.round(minute / step) * step) % 60;
		// Do we need to round the hour up?
		if( stepMinute === 0 && minute >= 30 ) {
			hour++;
			// Do we need to switch am/pm?
			if( hour === 12 || hour === 24 ) pm = !pm;
		}
		minute = stepMinute;
	}
	
	// Keep within range
	if( hour <= 0 || hour >= 24 ) hour = 0;
	if( minute < 0 || minute > 59 ) minute = 0;
 
	// Format output
	return (format || defaultFormat)
		// 12 hour without leading 0
        .replace(/g/g, hour === 0 ? '12' : 'g')
		.replace(/g/g, hour > 12 ? hour - 12 : hour)
		// 24 hour without leading 0
		.replace(/G/g, hour)
		// 12 hour with leading 0
		.replace(/h/g, hour.toString().length > 1 ? (hour > 12 ? hour - 12 : hour) : '0' + (hour > 12 ? hour - 12 : hour))
		// 24 hour with leading 0
		.replace(/H/g, hour.toString().length > 1 ? hour : '0' + hour)
		// minutes with leading zero
		.replace(/i/g, minute.toString().length > 1 ? minute : '0' + minute)
		// simulate seconds
		.replace(/s/g, '00')
		// lowercase am/pm
		.replace(/a/g, pm ? 'pm' : 'am')
		// lowercase am/pm
		.replace(/A/g, pm ? 'PM' : 'AM');
	
}


function update()...