Javascript Date parsing returning strange results in Chrome

Question

I observed some strange Date behaviour in Chrome (Version 74.0.3729.131 (Official Build) (64-bit)). Following javascript was executed in the Chrome Dev Console:

Answer 1

Okay, seems like this behaviour cannot be avoided, so you should parse dates manually. But the way to parse it is pretty simple.

If we are parsing date in ISO 8601 format, the mask of date string looks like this:

--
T::(.)?(Z|(+|-):)?

1. Getting date and time separately

The T in string separates date from time. So, we can just split ISO string by T

var isoString = `2019-05-09T13:26:10.979Z`
var [dateString, timeString] = isoString.split("T")

2. Extracting date parameters from date string

So, we have dateString == "2019-05-09". This is pretty simple now to get this parameters separately

var [year, month, date] = dateString.split("-").map(Number)

3. Handling time string

With time string we should make more complex actions due to its variability.
We have timeString == "13:26:10Z" Also it's possible timeString == "13:26:10" and timeString == "13:26:10+01:00

var clearTimeString = timeString.split(/[Z+-]/)[0]
var [hours, minutes, seconds] = clearTimeString.split(":").map(Number)

var offset = 0 // we will store offset in minutes, but in negation of native JS Date getTimezoneOffset
if (timeString.includes("Z")) {
    // then clearTimeString references the UTC time
    offset = new Date().getTimezoneOffset() * -1
} else {
    var clearOffset = timeString.split(/[+-]/)[1]
    if (clearOffset) {
        // then we have offset tail
        var negation = timeString.includes("+") ? 1 : -1 // detecting is offset positive or negative
        var [offsetHours, offsetMinutes] = clearOffset.split(":").map(Number)
        offset = (offsetMinutes + offsetHours * 60) * negation
    } // otherwise we do nothing because there is no offset marker
}

At this point we have our data representation in numeric format:
year, month, date, hours, minutes, seconds and offset in minutes.

4. Using ...native JS Date constructor

Yes, we cannot avoid it, because it is too cool. JS Date automatically match date for all negative and too big values. So we can just pass all parameters in raw format, and the JS Date constructor will create the right date for us automatically!

new Date(year, month - 1, date, hours, minutes + offset, seconds)

Voila! Here is fully working example.

function convertHistoricalDate(isoString) {
  var [dateString, timeString] = isoString.split("T")
  var [year, month, date] = dateString.split("-").map(Number)
  
  var clearTimeString = timeString.split(/[Z+-]/)[0]
  var [hours, minutes, seconds] = clearTimeString.split(":").map(Number)
  
  var offset = 0 // we will store offset in minutes, but in negation of native JS Date getTimezoneOffset
  if (timeString.includes("Z")) {
    // then clearTimeString references the UTC time
    offset = new Date().getTimezoneOffset() * -1
  } else {
    var clearOffset = timeString.split(/[+-]/)[1]
    if (clearOffset) {
      // then we have offset tail
      var negation = timeString.includes("+") ? 1 : -1 // detecting is offset positive or negative
      var [offsetHours, offsetMinutes] =   clearOffset.split(":").map(Number)
      offset = (offsetMinutes + offsetHours * 60) * negation
    } // otherwise we do nothing because there is no offset marker
  }

  return new Date(year, month - 1, date, hours, minutes + offset, seconds)
}

var testDate1 = convertHistoricalDate("1894-01-01T00:00:00+01:00")
var testDate2 = convertHistoricalDate("1893-01-01T00:00:00+01:00")
var testDate3 = convertHistoricalDate("1894-01-01T00:00:00-01:00")
var testDate4 = convertHistoricalDate("1893-01-01T00:00:00-01:00")

console.log(testDate1.toLocaleDateString(), testDate1.toLocaleTimeString())
console.log(testDate2.toLocaleDateString(), testDate2.toLocaleTimeString())
console.log(testDate3.toLocaleDateString(), testDate3.toLocaleTimeString())
console.log(testDate4.toLocaleDateString(), testDate4.toLocaleTimeString())

Note

In this case we are getting Date instance with all its own values (like .getHours()) being normalized, including timezone offset. The testDate1.toISOString will still return weird result. But if you are working with this date, it will probably 100% fit your needings.

Hope that helped :)