Javascript add leading zeroes to date

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The modern way

The new modern way to do this is to use toLocaleDateString, because it allows you not only to format a date with proper localization, but even to pass format options to achieve the desired result:

const date = new Date(2018, 2, 1)
const result = date.toLocaleDateString("en-GB", { // you can use undefined as first argument
  year: "numeric",
  month: "2-digit",
  day: "2-digit",
})
console.log(result) // outputs “01/03/2018”

Or using a Temporal object (still in proposal, caniuse):

const date = new Temporal.PlainDate(2018, 2, 1) // also works with zoned date
const result = date.toLocaleString("en-GB", { // you can use undefined as first argument
  year: "numeric",
  month: "2-digit",
  day: "2-digit",
})
console.log(result) // outputs “01/03/2018”

When you use undefined as the first argument it will detect the browser language, instead. Alternatively, you can use 2-digit on the year option, too.

Performance

If you plan to format a lot of dates, you should consider using Intl.DateTimeFormat instead:

const formatter = new Intl.DateTimeFormat("en-GB", { // <- re-use me
  year: "numeric",
  month: "2-digit",
  day: "2-digit",
})
const date = new Date(2018, 2, 1) // can also be a Temporal object
const result = formatter.format(date)
console.log(result) // outputs “01/03/2018”

The formatter is compatible with Date and Temporal objects.

Historical dates

Unlike in the Temporal constructor years between 0 and 99 will be interpreted as 20th century years on the Date constructor. To prevent this, initialize the date like so:

const date = new Date()
date.setFullYear(18, 2, 1) // the year is A.D. 18

This is not required for Temporal objects, but years below 1000 will not contain leading zeros in all cases, because the formatter (that is shared for the Date and Temporal API) does not support 4-digit formatting at all. In this case you have to do manual formatting (see below).

For the ISO 8601 format

If you want to get your date in the YYYY-MM-DD format (ISO 8601), the solution looks different:

const date = new Date(Date.UTC(2018, 2, 1))
const result = date.toISOString().split('T')[0]
console.log(result) // outputs “2018-03-01”

Your input date should be in the UTC format or toISOString() will fix that for you. This is done by using Date.UTC as shown above.

Historical dates for the ISO 8601 format

Unlike in the Temporal constructor years between 0 and 99 will be interpreted as 20th century years on the Date constructor. To prevent this, initialize the date like so to be used for the ISO 8601 format:

const date = new Date()
date.setUTCFullYear(18, 2, 1) // the year is A.D. 18

Note that the ISO format for Temporal objects with dates before the year 1000 or after the year 9999 will have a different formatting compared to the legacy Date API. It is recommend to fallback to custom formatting to enforce 4 digit years in all circumstances.

Custom 4-digit formatting on the year

Sadly, the formatter doesn’t support leading zeros on the year. There is no 4-digit option. This will remain for Temporal objects as well, because they do share the same formatter.

Fortunately, the ISO format of the Date API will always display at least 4 digits on the year, although Temporal objects do not. So at least for the Date API you can format historical dates before the year 1000 with leading zeros by falling back to a manual formatting approach using part of the ISO 8601 format method:

const date = new Date()
date.setUTCFullYear(18, 2, 1)
const ymd = date.toISOString().split('T')[0].split('-')
const result = `${ymd[2]}/${ymd[1]}/${ymd[0]}`
console.log(result) // outputs “01/03/0018”

For a Temporal object a different route is necessary, since the ISOYearString will be formatted differently for dates before the year 1000 and after the year 9999 as mentioned before:

const date = new Temporal.PlainDate(2018, 2, 1) // also works with zoned date
const zeroPad = (n, digits) => n.toString().padStart(digits, '0');
const result = `${zeroPad(date.day, 2)}/${zeroPad(date.month, 2)}/${zeroPad(date.year, 4)}`;
console.log(result) // outputs “01/03/0018”

Miscellaneous

For the Date API there is also toLocaleTimeString, that allows you to localize and format the time of a date.

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