How to parse a date in format "YYYYmmdd" in JavaScript?

Question

This is a noob question:

How to parse a date in format "YYYYmmdd" without external libraries ? If the input string is not in this format I would like to get invalid Date (or undefined if it will be easier).

Haunt

https://mcmap.net/q/36712/-why-does-date-parse-give-incorrect-results – Affiliate

Answer 1

function parse(str) {
    if(!/^(\d){8}$/.test(str)) return "invalid date";
    var y = str.substr(0,4),
        m = str.substr(4,2),
        d = str.substr(6,2);
    return new Date(y,m,d);
}

Usage:

parse('20120401');

UPDATE:

As Rocket said, months are 0-based in js...use this if month's aren't 0-based in your string

function parse(str) {
    if(!/^(\d){8}$/.test(str)) return "invalid date";
    var y = str.substr(0,4),
        m = str.substr(4,2) - 1,
        d = str.substr(6,2);
    return new Date(y,m,d);
}

UPDATE:

More rigorous checking for validity of date. Adopted HBP's way to validate date.

function parse(str) {
    var y = str.substr(0,4),
        m = str.substr(4,2) - 1,
        d = str.substr(6,2);
    var D = new Date(y,m,d);
    return (D.getFullYear() == y && D.getMonth() == m && D.getDate() == d) ? D : 'invalid date';
}

Longeron

Note: in JavaScript, months are zero-indexed, so you may want to use new Date(y, m-1, d);, otherwise 20120401 will be May 1st, not April 1st. – Fries

What if the input is 00005050? It is invalid string but new Date('0000', '50', 50') is Tue Jun 09 1903 00:00:00 GMT+0300 (IDT) – Haunt

all right...one more update coming up...if you still have problems, do tell them so that i can improve this answer...actually i did think of this, but thought this might be a trivial case, hence didn't add any code for detailed validation... – Longeron

@ParthThakkar Thanks. However now it gets complicated. I wonder if there is a simpler solution. – Haunt

All right...another update coming up...this time, I've adopted HBP's way to validate date, much simpler and easier to understand! An upvote to HBP! – Longeron

@ParthThakkar—note that the date can be validated by just checking the month (per HBP's answer). – Monge

Answer 2

A more robust version validating the numbers :

 function parse (str) {
        // validate year as 4 digits, month as 01-12, and day as 01-31 
        if ((str = str.match (/^(\d{4})(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])$/))) {
           // make a date
           str[0] = new Date (+str[1], +str[2] - 1, +str[3]);
           // check if month stayed the same (ie that day number is valid)
           if (str[0].getMonth () === +str[2] - 1)
              return str[0];
        }
        return undefined;
 }

See fiddle at : http://jsfiddle.net/jstoolsmith/zJ7dM/

I recently wrote a much more capable version you can find here : http://jsfiddle.net/jstoolsmith/Db3JM/

Terpsichorean

Great, but instead of returning undefined where parsing fails, it should return an invalid date, i.e. new Date(NaN). – Monge

To do this replace the return undefined with return date(NaN) – Terpsichorean

Answer 3

simplistic answer maybe, without checks, but fast...

var date = parseInt(date);
new Date(date / 10000, date % 10000 / 100, date % 100);

or, if months are not zero based in the source,

new Date(date / 10000, (date % 10000 / 100) - 1, date % 100);

Maurits

Answer 4

Using newer language features for destructuring assignment:

// Parse YYYYMMDD to Date
function parseYMD(s) {
  let [C,Y,M,D] = s.match(/\d\d/g);
  return new Date(C+Y, M-1, D);
}

['20211125', // Valid date
 '2021112',  // Invalid input
 '20219999'  // Invalid date components
].forEach(d => 
  console.log(`${d} -> ${parseYMD(d).toDateString()}`)
);

Note that this does not validate the date components, any string with 8 or more digits will produce a valid Date. Anything else will produce an invalid Date. If strict parsing is required, consider:

// Parse YYYYMMDD to Date. If any part is out of
// range, return an invalid Date.
function parseYMD(s) {
  let [C,Y,M,D] = s.match(/\d\d/g);
  let d = new Date(C+Y, M-1, D);
  let [yr,mo,da] = d.toLocaleDateString('en-CA').split(/\D/); 
  if (yr != C+Y || mo != M || da != D) {
    d.setTime(NaN);
  }
  return d;
}

['20211125', // Valid date
 '2021112',  // Invalid input
 '20219999'  // Invalid date components
].forEach(d => 
  console.log(`${d} -> ${parseYMD(d).toDateString()}`)
);

Monge

Answer 5

combining HBP's answer and this answer to get a function that parses YYYYMMDDHHmm and here is a fiddle

var parseTS=function(str){
                // validate year as 4 digits, month as 01-12, and day as 01-31
                if ((str = str.match (/^(\d{4})(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])([01]\d|2[0-3])([0-5]\d)$/))) {
                    // make a date
                    str[0] = new Date (+str[1], +str[2] - 1, +str[3], +str[4], +str[5]);
                    // check if month stayed the same (ie that day number is valid)
                    if (str[0].getMonth () === +str[2] - 1) {
                        return str[0];
                    }
                }
                return undefined;
            };

    console.log(parseTS('201501012645'));

Marj

Answer 6

This worked for me:

    function parse(str) {
      const year = str.substring(0, 4)
      const month = str.substring(4, 6)
      const day = str.substring(6, 8)
      return new Date(`${year}-${month}-${day}`)
    }

Gelderland