Locales can't be used in parsing dates

[raketenolli]

Update: This was originally a bug about missing documentation about using locales without babel.

Describe the bug
I was under the assumption that locales would help in parsing dates.

I tried

const dayjs = require('dayjs');
const enGB = require('dayjs/locale/en-gb');

const dateStringUS = "09/10/2019, 08:51";
const dateStringGB = "11/09/2019, 08:50";

var dateUS = dayjs(dateStringUS).toDate();
dayjs.locale(enGB);
var dateGB = dayjs(dateStringGB).toDate();

console.log("dateUS: " + dateUS);
console.log("dateGB: " + dateGB);

and I get

dateUS: Tue Sep 10 2019 08:51:00 GMT+0200 (GMT+02:00)
dateGB: Sat Nov 09 2019 08:50:00 GMT+0100 (GMT+01:00)

Expected behavior
I was expecting an output like

dateUS: Tue Sep 10 2019 08:51:00 GMT...
dateGB: Wed Sep 11 2019 08:50:00 GMT...

Using dayjs(datestring, { locale: enGB }); yielded the same result.

Information

• Day.js Version v1.8.16
• OS: Windows 10
• Browser node.js v10.15.3
• Time zone: n/a

[raketenolli]

So I tried

var dayjs = require('dayjs');
var LocalizedFormat = require('dayjs/plugin/localizedFormat');
var enGB = require('dayjs/locale/en-gb');

const dateStringUS = "09/10/2019, 08:51";
const dateStringGB = "11/09/2019, 08:50";

var dateUS = dayjs(dateStringUS).format("LLLL");
dayjs.locale(enGB);
var dateGB = dayjs(dateStringGB).format("LLLL");

console.log("dateUS: " + dateUS);
console.log("dateGB: " + dateGB);

and obtained

dateUS: Tuesday, September 10, 2019 8:51 AM
dateGB: Saturday, 9 November 2019 08:50

so the locales do work.

I was under the impression that parsing would also respect locales, but this doesn't seem to be the case?

[raketenolli]

It seems CustomParseFormat supports adding a parsing format such as

var dateGB = dayjs(dateStringGB, "DD/MM/YYYY, HH:mm");

but no locale-supported format string such as

...dayjs(dateStringGB, "L, LT");

which would be useful here because the command could be used regardless of the locale currently set.

[iamkun]

I did not get what you mean.

Using locale will localize the week name 'Saturday' to 'Saturday in another language', but will of course not display as another day like you expected 'Wed'

[raketenolli]

Sorry, at first I was confused about using locales without Babel (require vs. import), but it turns out my problems were based on my expectations about using locales for parsing the dates. If I pass "11/09/2019" to dayjs for parsing with locale en-gb, I would expect 2019-09-11 as the date, not 2019-11-09 as it happened. Turns out I needed to set a CustomParseFormat, but this one only allows non-locale placeholders for the format, as above, e.g. "DD/MM/YYYY", whereas I had hoped for something like "L", or not format string necessary at all.

[iamkun]

@raketenolli CustomParseFormat plugin supports tokens like 'Do' 'MMMM' which are all locale-aware.

[raketenolli]

I know that if you provide a string containing e.g. "März", it would be parsed properly to March if "MMMM" is provided as the format string for the CustomParseFormat and locale is set to "de". But the CustomParseFormat does not support the localized formats like "L", "LTS" etc. which would enable the user to just set the locale but not have to worry about the exact formatting if they know they can expect e.g. an "L, LTS" formatted string ("10/31/2019, 08:30 PM" vs. "31.10.2019, 20:30").

[iamkun]

Here's working around to get the locale formats
#680

[raketenolli]

Thanks, so a solution to my issue would be

var L = dayjs.Ls[dayjs.locale()].formats.L;
var LT = dayjs.Ls[dayjs.locale()].formats.LT;
var dateParsedUsingCurrentLocale = dayjs(dateString, `${L}, ${LT}`);
console.log(dateParsedUsingCurrentLocale.format("LLLL"));