What is the best way to parse a time into a Date o

Question

I am working on a form widget for users to enter a time of day into a text input (for a calendar application). Using JavaScript (we are using jQuery FWIW), I want to find the best way to parse the text that the user enters into a JavaScript Date() object so I can easily perform comparisons and other things on it.

I tried the parse() method and it is a little too picky for my needs. I would expect it to be able to successfully parse the following example input times (in addition to other logically similar time formats) as the same Date() object:

• 1:00 pm
• 1:00 p.m.
• 1:00 p
• 1:00pm
• 1:00p.m.
• 1:00p
• 1 pm
• 1 p.m.
• 1 p
• 1pm
• 1p.m.
• 1p
• 13:00
• 13

I am thinking that I might use regular expressions to split up the input and extract the information I want to use to create my Date() object. What is the best way to do this?

Answer 1

Don't bother doing it yourself, just use datejs.

Answer 2

I wasn't happy with the other answers so I made yet another one. This version:

• Recognizes seconds and milliseconds
• Returns undefined on invalid input such as "13:00pm" or "11:65"
• Returns a local time if you provide a localDate parameter, otherwise returns a UTC time on the Unix epoch (Jan 1, 1970).
• Supports military time like 1330 (to disable, make the first ':' required in the regex)
• Allows an hour by itself, with 24-hour time (i.e. "7" means 7am).
• Allows hour 24 as a synonym for hour 0, but hour 25 is not allowed.
• Requires the time to be at the beginning of the string (to disable, remove ^\s* in the regex)
• Has test code that actually detects when the output is incorrect.

Edit: it's now a package including a timeToString formatter: npm i simplertime

---

/**
 * Parses a string into a Date. Supports several formats: "12", "1234",
 * "12:34", "12:34pm", "12:34 PM", "12:34:56 pm", and "12:34:56.789".
 * The time must be at the beginning of the string but can have leading spaces.
 * Anything is allowed after the time as long as the time itself appears to
 * be valid, e.g. "12:34*Z" is OK but "12345" is not.
 * @param {string} t Time string, e.g. "1435" or "2:35 PM" or "14:35:00.0"
 * @param {Date|undefined} localDate If this parameter is provided, setHours
 *        is called on it. Otherwise, setUTCHours is called on 1970/1/1.
 * @returns {Date|undefined} The parsed date, if parsing succeeded.
 */
function parseTime(t, localDate) {
  // ?: means non-capturing group and ?! is zero-width negative lookahead
  var time = t.match(/^\s*(\d\d?)(?::?(\d\d))?(?::(\d\d))?(?!\d)(\.\d+)?\s*(pm?|am?)?/i);
  if (time) {
    var hour = parseInt(time[1]), pm = (time[5] || ' ')[0].toUpperCase();
    var min = time[2] ? parseInt(time[2]) : 0;
    var sec = time[3] ? parseInt(time[3]) : 0;
    var ms = (time[4] ? parseFloat(time[4]) * 1000 : 0);
    if (pm !== ' ' && (hour == 0 || hour > 12) || hour > 24 || min >= 60 || sec >= 60)
      return undefined;
    if (pm === 'A' && hour === 12) hour = 0;
    if (pm === 'P' && hour !== 12) hour += 12;
    if (hour === 24) hour = 0;
    var date = new Date(localDate!==undefined ? localDate.valueOf() : 0);
    var set = (localDate!==undefined ? date.setHours : date.setUTCHours);
    set.call(date, hour, min, sec, ms);
    return date;
  }
  return undefined;
}

var testSuite = {
  '1300':  ['1:00 pm','1:00 P.M.','1:00 p','1:00pm','1:00p.m.','1:00p','1 pm',
            '1 p.m.','1 p','1pm','1p.m.', '1p', '13:00','13', '1:00:00PM', '1300', '13'],
  '1100':  ['11:00am', '11:00 AM', '11:00', '11:00:00', '1100'],
  '1359':  ['1:59 PM', '13:59', '13:59:00', '1359', '1359:00', '0159pm'],
  '100':   ['1:00am', '1:00 am', '0100', '1', '1a', '1 am'],
  '0':     ['00:00', '24:00', '12:00am', '12am', '12:00:00 AM', '0000', '1200 AM'],
  '30':    ['0:30', '00:30', '24:30', '00:30:00', '12:30:00 am', '0030', '1230am'],
  '1435':  ["2:35 PM", "14:35:00.0", "1435"],
  '715.5': ["7:15:30", "7:15:30am"],
  '109':   ['109'], // Three-digit numbers work (I wasn't sure if they would)
  '':      ['12:60', '11:59:99', '-12:00', 'foo', '0660', '12345', '25:00'],
};

var passed = 0;
for (var key in testSuite) {
  let num = parseFloat(key), h = num / 100 | 0;
  let m = num % 100 | 0, s = (num % 1) * 60;
  let expected = Date.UTC(1970, 0, 1, h, m, s); // Month is zero-based
  let strings = testSuite[key];
  for (let i = 0; i < strings.length; i++) {
    var result = parseTime(strings[i]);
    if (result === undefined ? key !== '' : key === '' || expected !== result.valueOf()) {
      console.log(`Test failed at ${key}:"${strings[i]}" with result ${result ? result.toUTCString() : 'undefined'}`);
    } else {
      passed++;
    }
  }
}
console.log(passed + ' tests passed.');