Converting milliSeconds to SRT time using JS

Question

I have an input like like this:

start: 0.21 | duration: 0.30 | text: Subtitle Text 1 
start: 0.32 | duration: 0.52 | text: Subtitle Text 2 

This input needs to be converted into SRT format, so it becomes something like this:

1
00:00:00,210 --> 00:00:00,300
Subtitle Text 1

2
00:00:00,320 --> 00:00:00,520
Subtitle Text 2

JS:

function formatMilliseconds($milliseconds) {
    $seconds = Math.floor($milliseconds / 1000);
    $minutes = Math.floor($seconds / 60);
    $hours = Math.floor($minutes / 60);
    $milliseconds = $milliseconds % 1000;
    $seconds = $seconds % 60;
    $minutes = $minutes % 60;
console.log( $hours, $minutes, $seconds, $milliseconds); // 0 0 0 0.21

}

formatMilliseconds(0.21)

Answer 1

First you'll want to bring your input format into a processable format, something along the lines of

var subtitles = [
{
  start: 0.21,
  end: 0.3,
  text: "Subtitle Text 1"
},
{
  start: 0.32,
  end: 0.52,
  text: "Subtitle Text 2"
}
];

Note that I chose "end" instead of "duration" here, because "duration" in my eyes would mean that the subtitle ends at start + duration (0.21 + 0.3 = 0.51) seconds.

The formatting function was almost correct, but needed the input value supplied as milliseconds instead of decimal second values. For output, you also need to pad hours/minuts/seconds with zeroes. You'd better use a helper function for this where you can specify the target string length, but for this example I just did it inline:

var subtitles = [
    {
      start: 0.21,
      end: 0.3,
      text: "Subtitle Text 1"
    },
    {
      start: 0.32,
      end: 0.52,
      text: "Subtitle Text 2"
    }
  ],
  srtCount = 0;

function srtTimestamp(seconds) {
    var $milliseconds = seconds*1000;
    
    $seconds = Math.floor($milliseconds / 1000);
    $minutes = Math.floor($seconds / 60);
    $hours = Math.floor($minutes / 60);
    $milliseconds = $milliseconds % 1000;
    $seconds = $seconds % 60;
    $minutes = $minutes % 60;
    return ($hours < 10 ? '0' : '') + $hours + ':'
         + ($minutes < 10 ? '0' : '') + $minutes + ':'
         + ($seconds < 10 ? '0' : '') + $seconds + ','
         + ($milliseconds < 100 ? '0' : '') + ($milliseconds < 10 ? '0' : '') + $milliseconds;
}

function inputToSRT(sub_in) {
  return ++srtCount + "\r\n" + srtTimestamp(sub_in.start) + " --> " + srtTimestamp(sub_in.end) + "\r\n" + sub_in.text + "\r\n\r\n";
}
  
for (var i=0; i<subtitles.length; i++) {
  console.log(inputToSRT(subtitles[i]));
}

Answer 2

You can implement such conversion by using moment.js for date/time manipulations and sprintf for strings formatting

var data = 'start: 0.21 | duration: 0.30 | text: Subtitle Text 1' + '\n' +
    'start: 0.32 | duration: 0.52 | text: Subtitle Text 2';

function formatSrt(data) {
    var lines = data.split('\n');
    var result = [];
    var formatTime = function (value) {
        if (typeof value === 'string') {
            value = parseFloat(value);
            if (isNaN(value)) {
                throw new Exception('Invalid time "' + value + '"');
            }
        }
        var d = moment.duration(value * 1000, 'ms');
        return sprintf('%02d:%02d:%02d,%03d', d.get('h'), d.get('m'), d.get('s'), d.get('ms'));
    }
    lines.forEach(function (line, index) {
        result.push(index + 1);
        var parts = line.split('|');
        var start = formatTime(parts.shift().split(':').pop().trim());
        var end = formatTime(parts.shift().split(':').pop().trim());
        var comment = parts.shift().split(':').pop().trim();
        result.push(sprintf('%s --> %s', start, end));
        result.push(comment);
        result.push('');
    })
    return result.join('\n');
}

console.log(formatSrt(data));

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.19.1/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/sprintf/1.1.1/sprintf.min.js"></script>