Date Format Yyyy-Mm-Ddthh:Mm:Ssz

Java SimpleDateFormat(yyyy-MM-dd'T'HH:mm:ss'Z') gives timezone as IST

You haven't set the timezone only added a Z to the end of the date/time, so it will look like a GMT date/time but this doesn't change the value.

Set the timezone to GMT and it will be correct.

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");
sdf.setTimeZone(TimeZone.getTimeZone("GMT"));

Change date from MM/dd to yyyy-MM-dd'T'HH:mm:ss.SSZ in Java

In the question, the date format pattern indicates a desire for 2-digit fractional seconds. SimpleDateFormat cannot do that.

The newer Java 8 Time API can, and you should be using that anyway.

If you're running on Java 6 or 7, get the ThreeTen-Backport library.

To parse a MM/dd formatted string and get a full timestamp with current year and time-of-day, in the default time zone, use the following code:

String strDate = "06/05";
MonthDay monthDay = MonthDay.parse(strDate, DateTimeFormatter.ofPattern("MM/dd"));
ZonedDateTime date = ZonedDateTime.now().with(monthDay);
System.out.println(date.format(DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSZ")));

Sample Output

2020-06-05T14:52:48.45-0400

date format yyyy-MM-ddTHH:mm:ssZ

Using UTC

ISO 8601 (MSDN datetime formats)

Console.WriteLine(DateTime.UtcNow.ToString("s") + "Z");

2009-11-13T10:39:35Z

The Z is there because

If the time is in UTC, add a 'Z'
directly after the time without a
space. 'Z' is the zone designator for
the zero UTC offset. "09:30 UTC" is
therefore represented as "09:30Z" or
"0930Z". "14:45:15 UTC" would be
"14:45:15Z" or "144515Z".

If you want to include an offset

int hours = TimeZoneInfo.Local.BaseUtcOffset.Hours;
string offset = string.Format("{0}{1}",((hours >0)? "+" :""),hours.ToString("00"));
string isoformat = DateTime.Now.ToString("s") + offset;
Console.WriteLine(isoformat);

Two things to note: + or - is needed after the time but obviously + doesn't show on positive numbers. According to wikipedia the offset can be in +hh format or +hh:mm. I've kept to just hours.

As far as I know, RFC1123 (HTTP date, the "u" formatter) isn't meant to give time zone offsets. All times are intended to be GMT/UTC.

Understanding specific UTC time format YYYY-MM-DDTHH:MM:SS.SSSZ

Firstly please have a read of the iso8601 information. It's becoming more common place to deal with times in different time zones (e.g. server time zone and client time zone) and the standard is really useful.

In particular please read about UTC or "Zulu" time here.

The program is correct, since the London time is one hour ahead of "UTC" time in summer
The trailing 'Z' is a short notation for UTC (Zulu). You could also write "+00:00" instead of 'Z'. The SS.SSS refer to seconds and milliseconds - not related to the time zone. In devnull's comment he shows you how to apply an offset for summer.

Edit:

There's been some discussion in the comments about whether iso8601 timezone includes timezone or not, and whether timezone will in fact be printed out.

This depends completely on the date/time implementation. If we are using SimpleDateFormat then timezone is supported and will be printed.

Here's a code example to illustrate

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");
formatter.setTimeZone(TimeZone.getTimeZone("UTC"));
System.out.println(formatter.format(new Date()));
formatter.setTimeZone(TimeZone.getTimeZone("Europe/London"));
System.out.println(formatter.format(new Date()));

Output

2016-06-02T12:53:14.924Z
2016-06-02T13:53:14.925+01:00

Naturally, if you are using a different date/time library such as joda-time, then the implentation details will be different.

Edit: As @DerrylThomas pointed out with SimpleDateFormat wise to use lower case y for years - unless it's intended to use week year - explained in a bit of detail in another answer to a similar question https://stackoverflow.com/a/56911450.

Python 3 How to format to yyyy-mm-ddThh:mm:ssZ

Try datetime library

import datetime

output_date = datetime.datetime.now().strftime("%Y-%m-%dT%H:%M:%SZ")
print(output_date)

For more information, refer to the Python Documentation.

Why I can't parse this date format yyyy-MM-dd'T'HH:mm:ss.SSSZ?

You should use Z the same way you use T for the parser to recognize the character in format

val stringDate = "2021-12-16T16:42:00.000Z"
val sdf = SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'")
var consultationDate = sdf.parse(stringDate)

Not able to understand YYYY-MM-DDTHH:MM:SS date format

    LocalDateTime dateTime = LocalDateTime.parse("2018-01-30T23:59:59.000");
    System.out.println(dateTime);

This prints:

2018-01-30T23:59:59

Your string is in ISO 8601 format. UTC or Coordinated Universal Time is not a format, it is a standard time used to define the time the rest of use in our respective time zones.

The date-time classes you were using, SimpleDateFormat and Date, are long outdated and the former in particular notoriously troublesome. I recommend that you instead use java.time, the modern Java date and time API. It is so much nicer to work with.

A LocalDateTime is a date with time of day and without time zone or offset from UTC. Its one-argument parse method parses ISO 8601, which is why no explicit formatter is needed.

What went wrong in your code

Your format pattern string has a number of issues to it. Which is one reason why you should appreciate the above solution without any explicit formatter. The first thing that goes wrong is: Your format pattern string has a colon, :, between seconds and milliseconds, whereas your date-time string has a dot, .. This is why you get the exception.

However, fixing this, your code yields the following Date:

Sun Dec 31 23:00:00 CET 2017

It’s one month off from the expected, and the minutes and seconds are missing. Because:

Uppercase YYYY is for week-based year and only useful with a week number. You need lowercase yyyy for year.
Uppercase DD is for day of year. You need lowercase dd for day of month.
You correctly used uppercase MM for month. Trying the same again for minutes won’t work. Maybe you can guess by now: it’s lowercase mm.
Not surprising you need lowercase ss for seconds.
UsingMS for milliseconds is interesting. SimpleDateFormat takes it as M for month (which we’ve already had twice before) and uppercase S for millisecond. Instead you needed uppercase SSS for the three digits of milliseconds.

Java format yyyy-MM-dd'T'HH:mm:ss.SSSz to yyyy-mm-dd HH:mm:ss

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
SimpleDateFormat output = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date d = sdf.parse(time);
String formattedTime = output.format(d);

This works. You have to use two SimpleDateFormats, one for input and one for output, but it will give you just what you are wanting.