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Error while Converting specific date time to Sydney date using c# code

Using below code I am trying to convert specific date time to Sydney date time.
string datetime = "20200424-04:09:42.145";
datetime = datetime.Replace("-", " ").Insert(4, "-").Insert(7, "-");
TimeZoneInfo dest = TimeZoneInfo.FindSystemTimeZoneById("AUS Eastern Standard Time");
TimeZoneInfo src = TimeZoneInfo.FindSystemTimeZoneById("Greenwich Standard Time");
DateTime convertTime = TimeZoneInfo.ConvertTimeToUtc(Convert.ToDateTime(datetime), src);
DateTime transactTime = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(convertTime, dest.Id);
Getting output invalid format - 4/24/2020 2:09:42 PM.145
System format date format dd/mm/yyyy and time in 24 hours

A few things:
That input format is non-standard and a bit strange. If you can change wherever it is constructed, that would be a better approach. In the meantime, I suggest you parse it with DateTime.ParseExact instead of replacing characters to get Convert.ToDateTime to recognize it.
Greenwich Standard Time is the local time zone identifier for Monrovia (Liberia) and Reykjavik (Iceland) (and a few others). I suspect you are actually trying to convert from GMT/UTC to Sydney time. If so, you only need one conversion function - ConvertTimeFromUtc.
You say you're getting invalid output format, but you don't show how you create that. I assume you are doing something like Console.WriteLine(transactTime), or just putting transactTime in some other place that converts it to a string. When doing so, it will use the general format controlled by the current culture. (See the Remarks section in the DateTime.ToString documentation.)
It sounds like instead you would like a specific format, which you can get by specifying the desired output in the ToString method. You can either specify a standard format token (usually used with the current culture), or your own custom formatting tokens (usually used with the InvariantCulture).
A complete example illustrating the above points:
// Parse the input string to a DateTime, from a given format
DateTime dt = DateTime.ParseExact("20200424-04:09:42.145", "yyyyMMdd-HH:mm:ss.fff", CultureInfo.InvariantCulture);
// Convert the datetime from UTC to Sydney time
TimeZoneInfo dest = TimeZoneInfo.FindSystemTimeZoneById("AUS Eastern Standard Time");
DateTime transactTime = TimeZoneInfo.ConvertTimeFromUtc(dt, dest);
// Create and output the string you want to output, in a specific format
string output = transactTime.ToString("yyyy-MM-dd HH:mm:ss.fff", CultureInfo.InvariantCulture);
Console.WriteLine(output);
// Output: "2020-04-24 14:09:42.145"
(Working .NET Fiddle here.)

DateTime.ToString() not converting time

Looks like time is automatically getting changed during conversion.
My input is 17:15:25. However, it gets converted to 13:15:25
What could be the reason?
string testDate = Convert.ToDateTime("2016-03-24T17:15:25.879Z")
.ToString("dd-MMM-yyyy HH:mm:ss", CultureInfo.InvariantCulture);
The result I get for testDate is : 24-Mar-2016 13:15:25

The Z in your input indicates a UTC time, but the default behaviour of Convert.ToDateTime is to convert the result to your local time. If you look at the result of Convert.ToDateTime("2016-03-30T17:15:25.879Z").Kind you'll see it's Local.
I would suggest using DateTime.ParseExact, where you can specify the exact behaviour you want, e.g. preserving the UTC time:
var dateTime = DateTime.ParseExact(
"2016-03-30T17:15:25.879Z",
"yyyy-MM-dd'T'HH:mm:ss.FFF'Z'",
CultureInfo.InvariantCulture,
DateTimeStyles.AdjustToUniversal | DateTimeStyles.AssumeUniversal);
Console.WriteLine(dateTime); // March 30 2016 17:15 (...)
Console.WriteLine(dateTime.Kind); // Utc
You can then convert that value to a string however you want to.
Of course I'd really suggest using my Noda Time project instead, where you'd parse to either an Instant or a ZonedDateTime which would know it's in UTC... IMO, DateTime is simply broken, precisely due to the kind of problems you've been seeing.

When you use Convert.ToDateTime (which uses DateTime.Parse internally) with Z (which means Zulu time), this method adds your current time zone offset to that DateTime value.
Looks like your current time zone is UTC -04:00 right now and that's why method returns 4 hours back as a result.
I would suggest to use DateTime.ParseExact with AdjustToUniversal and AssumeUniversal styles for prevent Kind conversion as Jon answered.
From AdjustToUniversal
Date and time are returned as a Coordinated Universal Time (UTC). If
the input string denotes a local time, through a time zone specifier
or AssumeLocal, the date and time are converted from the local time to
UTC. If the input string denotes a UTC time, through a time zone
specifier or AssumeUniversal, no conversion occurs. If the input
string does not denote a local or UTC time, no conversion occurs and
the resulting Kind property is Unspecified.

Because of the CultureInfo.InvariantCulture.
You are converting a date in your GMT
Convert.ToDateTime("2016-03-24T17:15:25.879Z")
And then you are converting it to string in an invariant culture
ToString("dd-MMM-yyyy HH:mm:ss",CultureInfo.InvariantCulture);
You should use DateTime.ParseExact, and then use the invariant culture in the conversion.

Why does c# DateTime.Parse seem to interpret string incorrectly?

Can someone please explain why the following code outputs "4/14/2013 8:00:00 PM"?
var dt = "2013-04-15+00:00";
var result = DateTime.Parse(dt);

There are many ways of formatting dates/times in different regions, cultures, contexts, etc. When using DateTime.Parse it will do its best to guess what to do, but it will often be unsuccessful in cases where there are ambiguities in determining which datetime format is appropriate.
You can use DateTime.ParseExact in order to specify the exact formatting that the string is using to format the date.

At a guess, I think Anthony Pergram's comment is right. Most likely it is interpreting the string as a date, "2013-04-15", a timezone of "+00:00" which is GMT, and no time-of-day. The default time-of-day is midnight, so the resulting date is equal to "2013-04-15 at midnight GMT". That is then converted to your local timezone, which is four hours behind GMT, and output as you see.
If you can, you should use a more precise date/time format such as ISO 8601, which would look like "2013-04-15T00:00:00Z", or "2013-04-15T00:00:00-04:00"

The MSDN documentation describes this in depth.
http://msdn.microsoft.com/en-us/library/1k1skd40(v=vs.90).aspx
It is reading in the string in UTC because of the +00:00. It is printing out in localtime.

That is because it is taking your system timezone into account. It then adjust the specified time appropriately

The "+00:00" part of your string is interpreted as the time zone. That is a "geographical" zone of 0 hours and 0 minutes to the east of Greenwich.
If you intended the "+00:00" part to be the time of day instead, use a format string, like so:
var dt = "2013-04-15+00:00";
var result = DateTime.ParseExact(dt, "yyyy-MM-dd+HH:mm", CultureInfo.InvariantCulture);
The "HH" and "mm" means hours and minutes of the (local) time of day. In the opposite direction, with "yyyy-MM-ddzzz", the "+00:00" part will mean time zone.

C# datetime parse issue

When trying to convert date/time from string to DateTime, I'm not getting the correct value.
DateTime testDate = DateTime.ParseExact("2012-08-10T00:51:14.146Z", "yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal);
And my result is 2012-08-09 8:51:14 PM. Why is it being offset? I just want it to be the same value going in.

You are parsing the UTC date but the DateTime.Kind is local.
You should parse with DateTimeStyles.AdjustToUniversal to mark the Kind as Utc.
DateTime testDate = DateTime.ParseExact("2012-08-10T00:51:14.146Z", "yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal);
Trace.WriteLine(testDate); // 8/9/2012 8:51:14 PM
Trace.WriteLine(testDate.ToString()); // 8/9/2012 8:51:14 PM
Trace.WriteLine(testDate.ToUniversalTime()); // 8/10/2012 12:51:14 AM
Trace.WriteLine(testDate.Kind); // Local
testDate = DateTime.ParseExact("2012-08-10T00:51:14.146Z", "yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal);
Trace.WriteLine(testDate);// 8/10/2012 12:51:14 AM
Trace.WriteLine(testDate.ToString());// 8/10/2012 12:51:14 AM
Trace.WriteLine(testDate.ToUniversalTime());// 8/10/2012 12:51:14 AM
Trace.WriteLine(testDate.Kind); // Utc

Aware this is an answer many years later, but came across this today and once I worked out my problem wanted to add some context I didn't see in other answers.
Going back to the OPs code snippet the reason it doesn't do what the OP expected of taking a UTC time string and storing it as a UTC DateTime is because the DateTimeStyles.AssumeUniversal only specifies that the input string is a UTC string. By default C# will create DateTime's as a DateTimeKind.Local. This was pointed out in another answer. This means the time is converted from UTC to Local time.
To make sure that your end result ends up being a UTC DateTime you need to use the DateTimeStyles of DateTimeStyles.AdjustToUniversal. This was also mentioned in other answers. However, if your input string doesn't have an obvious timezone then it may be assumed to be local and then converted from Local to UTC.
Luckily DateTimeStyles is actually a flag enum meaning we can use both the above options at the same time. E.g:
DateTime testDate = DateTime.ParseExact("2012-08-10T00:51:14.146Z", "yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal | DateTimeStyles.AssumeUniversal);

You should use DateTimeStyles.AdjustToUniversal. The input DateTime is already universal, and the AdjustToUniversal enum option will convert the input to local time though you will get a resultant Kind of DateTimeKind.Unspecified.

What is your server timezone, if you use AssumeUniversal it will convert your input time to UTC time.
You probably in EST then.

I propose simply that you want to use .AssumeLocal instead of .AssumeUniversal.
You have a time stamp with unknown time zone, and if you know that the time stamp refers to an event that happened in your local time zone, then you should tell the parse to assume that the time stamp is local to you (i.e. in your time zone).
By using .AssumeUniversal, you are instructing the parser to treat the time stamp as if it was a UTC time stamp, which when you display it using your local time zone, it's automatically offset by that amount.
Edit:
One important thing: The capital "Z" in the time stamp suggests it is a UTC time stamp, which means you do want to treat it as Universal. If you want to treat it as a local time stamp, you should remove the Z from the time stamp and the corresponding parse string.
Reference: http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx#KSpecifier

Parsing a date time from an xml document with timezone abbreviations (e.g. CET )

I rarely parse xml files and when I try to use the features integrated in Linq2Xml. However today I stumbled upon a date format I am not able to cast via:
XElement element = ...
var date = (DateTime)element;
The date is in the following format:
01/28/2009 02:31:54 CET
I already tried to parse the date using TryParse in various combinations, cultures, but whatever format I try the datetime parser complains about the space at position 20. Is there a way to parse this date without splitting the string or doing awkward things?

It sounds like the timezone abbreviations (e.g. CET) aren't recognized. Do you know the offset?
This person is having a related problem:
Parse DateTime with time zone of form PST/CEST/UTC/etc
An example from his post:
DateTime dt1 = DateTime.ParseExact("24-okt-08 21:09:06 CEST".Replace("CEST", "+2"), "dd-MMM-yy HH:mm:ss z", culture);
Obviously your date time format is a bit different, but it's an idea to bail you out.
If you can, ask the person giving you the XML to generate the date/times with ISO 8601 format to avoid custom parsing.