How to determine date format from string?

Question

This question was already asked today but the owner seems have deleted it(even it has 4 up vote). However the question was so interesting, I have decided to post by my self again.

I have an object in Javascript which has one property with a date string.

Now I want set a new datetime to that property, but how can I set the new data without knowing its format?

The sample datetime looks like this "2018-01-01T20:09:00"

This question can be divided in 2 answer.

1. Identify the current mentioned format & set the the same format to object property.(This seems to be achieved easily if someone say what type of datetime format is this)

2. Identify some generic solution that determine any datetime format & convert given datetime to set object property.

Answer 1

If you can use moment.js then the following does the job:

var dateFormats = {
  "iso_int" : "YYYY-MM-DD",
  "short_date" : "DD/MM/YYYY",
  "iso_date_time": "YYYY-MM-DDTHH:MM:SS",
  "iso_date_time_utc": "YYYY-MM-DDTHH:MM:SSZ"
   //define other well known formats if you want
}

function getFormat(d){
  for (var prop in dateFormats) {
        if(moment(d, dateFormats[prop],true).isValid()){
           return dateFormats[prop];
        }
  }
  return null;
}

var dateInput = "2018-01-01T20:09:00";

var formatFound = getFormat(dateInput); //returns "YYYY-MM-DDTHH:MM:SS"
if(formatFound !==null){
   //do stuff
}

Check the moment js docs for more info on the supported dateFormats by default and populate your dateFormats object with them.

Answer 2

Tl;Dr

You can't do this it's impossible.

---

There's a lot of snake oil getting sold here.

Date formats are not mutually exclusive, two (or more) formats may look the same but the values represent different parts of a date. This makes it practically impossible to determine the date format from a string.

For example, 01/02/2022 is 1st Feb 2022 in UK format and 2nd Jan 2022 in US format. So given the string 01/02/2022 you cannot determine whether this is uk or us format.

Given an imaginary method returnFormat("01/02/2022") what would you expect the response to be here? It might return en-US or en-GB or an of the other overlapping formats.

Date formats were not designed with computer parsing in mind. The only date format designed for computers is a ISO 8601 format (e.g. 2008-05-11T15:30:00Z) which can include the UTC offset. so times with a Z are utc and +01:00 would be UTC +1. This format is specifically chosen so as not to clash with any human readable formats for the problems outlined above.