Importing date without local time transformation - node.js

Question

I am getting a date in DD-MMM-YY format, and am using dayjs to convert it to standard date format in nodejs.
However, the returned date is 1 day earlier than the input date.
I was told this is probably due to some difference in the server local time.
I can easily just add a day to the date, but as this will be a global function that works in multiple time zones, I just want to get the date "as is" without any automatic adjustments.
This is my function:
const convertDate = (date,format,zone) => {
dayjs.tz.setDefault(zone);
console.log(date);
console.log(dayjs(date));
console.log(dayjs.utc(date).$d);
console.log(dayjs.tz(date,format,zone).$d);
var newDate = dayjs.tz(date,zone);
//newDate.setDate(newDate.getDate());
return newDate;
}
No matter which methods I use or which zone I set, the date comes out as one day earlier than the input date.
For example, for a date of 01-APR-03 I get:
2003-03-31T21:00:00.000Z
I want the time to just be 2003-04-01T00:00:00.000Z.
Following comments, I have tried the following approach, but the result is the same:
const fixMonthName = (s) => s.replace(/[A-Z]{2}-/, (m) => m.toLowerCase());
const d = dayjs.utc(fixMonthName("22-FEB-02"), "DD-MMM-YY");
console.log(d);
const s = d.toISOString();
console.log(s); //{result:
M {
'$L': 'en',
'$u': true,
'$d': 2002-02-21T22:00:00.000Z,
'$x': {},
'$y': 2002,
'$M': 1,
'$D': 21,
'$W': 4,
'$H': 22,
'$m': 0,
'$s': 0,
'$ms': 0
}}
2002-02-21T22:00:00.000Z

Answer 1

Let's recap the problem:
You have a date-only string value of 01-APR-03 (equivalent to 2003-04-01).
You're then parsing it as timestamp, treating it as if it were 2003-04-01T00:00:00.000 (local time). This is the cause of the logical error.
Then you're looking at a UTC representation (2003-03-31T21:00:00.000Z in your example), and wondering why it's been shifted. (Z means UTC)
Fundamentally, a date and a timestamp are two different concepts. If you conflate them, you will have complications in your code such as the one you described.
A date can be thought of as a half-open range of timestamps (from the start of one day, to just before the start of the next). In other words, logically the following is true:
'2003-04-01' == ['2003-04-01T00:00:00.000', '2003-04-02T00:00:00.000')
If you parse a date-only value to an object that represents a timestamp, you are choosing to assign a point-in-time within that range. Thus, if you pick the very start of the range, you can easily shift into a different day when viewing that from another time zone.
Note that the JavaScript Date object is misnamed. It isn't a date, it's a timestamp.
A day.js object also represents a timestamp, as do most other libraries including Moment, Luxon, date-fns, and many others.
There are a few different solutions to this problem:
You can pick a time in the middle of the range which is less likely to be shifted to a different date when viewed from another time zone. For example, 12:00:00 noon. (Though this isn't perfect, as there are some time zones that go up to UTC+14.)
You can avoid treating a date as a timestamp, by keeping it in an object or string that represents it as a whole date.
Unfortunately, this isn't a concept that has caught on well in JavaScript yet. The language and most libraries do not handle it this way. (One notable exception is js-joda, which has a LocalDate data type.) However, this will eventually be coming to the JavaScript language itself via the Temporal proposal, which adds Temporal.PlainDate.
You can ignore the time portion of a timestamp and only look at the date part, but this only works if you lock all your operations to UTC rather than local time. In other words, treat '2003-04-01' as if it were '2003-04-01T00:00:00.000Z' and never convert it to local time or another time zone.
If you were using just the JavaScript Date object, then you would do:
const d = new Date('2003-04-01T00:00:00.000Z'); // the Z parses as UTC
const s = d.toISOString(); // this always emits UTC
But since you have a custom date format to parse and want to use day.js, you can do something like the following:
Define a function to work around day.js's parsing case sensitivity issue. (You need Apr, not APR.)
const fixMonthName = (s) => s.replace(/[A-Z]{2}-/, (m) => m.toLowerCase());
Parse the input string using day.js's UTC mode
const d = dayjs.utc(fixMonthName('01-APR-03'), 'DD-MMM-YY');
Get the output as a string however you would like, using any of day.js's display functions:
const s = d.toISOString(); // "2003-04-01T00:00:00.000Z"
// or
const s = d.format('YYYY-MM-DD'); // "2003-04-01"
Note that if you need a JavaScript Date object, do not use $d but instead call .toDate(). From there, make sure you are only using the UTC representation of the Date object. Keep in mind that while some environments will emit UTC when logging a Date object to the console (as if you called .toISOString(), other environments will emit the local time equivalent (as if you called .toString().