Chrome interprets ISO time without Z as UTC; C# issue

Should we just append Z to them on the server side?

TL;DR Yes, you probably should.

Correct handling of dates and date/times without a timezone has, sadly, varied through the years — both in terms of specification and JavaScript engines adherence to the specification.

When this answer was originally written in 2013, the ES5 spec (the first to have a defined date/time format for JavaScript, which was meant to be a subset of ISO-8601) was clear: No timezone = UTC:

The value of an absent time zone offset is “Z”.

But that was at odds with ISO-8601, in which the absense of a timezone indicator means "local time." Some implementations never implemented ES5's meaning, sticking instead to ISO-8601.

In ES2015 (aka "ES6"), it was changed to match ISO-8601:

If the time zone offset is absent, the date-time is interpreted as a local time.

However, this caused incompatibility problems with existing code, particularly with date-only forms like 2018-07-01, so in ES2016 it was changed yet again:

When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.

So new Date("2018-07-01") is parsed as UTC, but new Date("2018-07-01T00") is parsed as local time.

It's been consistent since, in ES2017 and in the upcoming ES2018; here's the link to the current editor's draft, which no longer has that exact text but still defines it the same way (though IMHO less clearly).

You can test your current browser here:

function test(val, expect) {
  var result = +val === +expect ? "Good" : "ERROR";
  console.log(val.toISOString(), expect.toISOString(), result);
}
test(new Date("2018-07-01"), new Date(Date.UTC(2018, 6, 1)));
test(new Date("2018-07-01T00:00:00"), new Date(2018, 6, 1));

Status as of September 2021:

• Modern versions of Firefox, Chrome, and Safari get this right, including iOS browsers (which all currently use Apple's JavaScriptCore JavaScript engine since Apple won't let them use their own if they do JIT)
• IE11 gets it right (interestingly)

Status as of April 2018:

• IE11 gets it right (interestingly)
• Firefox gets it right
• Chrome 65 (desktop, Android) gets it right
• Chrome 64 (iOS v11.3) gets the date/time form wrong (parses as UTC)
• iOS Safari in v11.3 gets the date/time form wrong (parses as UTC)

Oddly, I can't find an issue in the v8 issues list that's been fixed between v6.4 (the v8 in Chrome 64) and v6.5 (the v8 in Chrome 65); I can only find this issue which is still open, but which appears to have been fixed.

I ran into this problem, and interpreting the date with the local timezone made a lot more sense than changing to "Z", at least for my application. I created this function to append the local timezone info when it is missing in the ISO date. This can be used in place of new Date(). Partially derived from this answer: How to ISO 8601 format a Date with Timezone Offset in JavaScript?

parseDate = function (/*String*/ d) {
    if (d.search(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}$/) == 0) {
        var pad = function (num) {
            norm = Math.abs(Math.floor(num));
            return (norm < 10 ? '0' : '') + norm;
        },
        tzo = -(new Date(d)).getTimezoneOffset(),
        sign = tzo >= 0 ? '+' : '-';
        return new Date(d + sign + pad(tzo / 60) + ':' + pad(tzo % 60));
    } else {
        return new Date(d);
    }
}

David Hammond's answer is great but doesn't play all the tricks; so here is a modified version:

• allows for fractional part in date/time string
• allows for optional seconds in date/time string
• considers crossing over daylight-saving time

appendTimezone = function (/*String*/ d) {
    // check for ISO 8601 date-time string (seconds and fractional part are optional)
    if (d.search(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}(?::\d{2}(?:\.\d{1-3})?)?$/) == 0) {
        var pad = function (num) {
            norm = Math.abs(Math.floor(num));
            return (norm < 10 ? '0' : '') + norm;
        },
        tzo = -new Date(d).getTimezoneOffset(),
        sign = tzo >= 0 ? '+' : '-';

        var adjusted = d + sign + pad(tzo / 60) + ':' + pad(tzo % 60);

        // check whether timezone offsets are equal;
        // if not then the specified date is just within the hour when the clock
        // has been turned forward or back
        if (-new Date(adjusted).getTimezoneOffset() != tzo) {
            // re-adjust
            tzo -= 60;
            adjusted = d + sign + pad(tzo / 60) + ':' + pad(tzo % 60);
        }

        return adjusted;
    } else {
        return d;
    }
}

parseDate = function (/*String*/ d) {
    return new Date(appendTimezone(d));
}

In addition to @tig's answer (which was exactly what I was looking for):

Here's the solution for .NetCore 1

services.AddMvc();
services.Configure<MvcJsonOptions>(o =>
{
    o.SerializerSettings.DateTimeZoneHandling = DateTimeZoneHandling.Utc;
});

or

services.AddMvc().AddJsonOptions(o => o.SerializerSettings.DateTimeZoneHandling = DateTimeZoneHandling.Utc);

For .NetCore 1.0.1

services
    .AddMvcCore()
    .AddJsonFormatters(o => o...);