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How to globally fix the timezone in android app?

I am looking for the most simple and cleanest way to fix the timezone for all dates in an Android app. The idea is to have the app running as if the user were in another timezone. Let me clarify what I am looking for:
Let's say the user's phone is set to America/New_York then I would like my app to show all dates (are in UTC) in the Europe/Amsterdam timezone, regardless of the timezone that is set on the phone itself. And if I make a comparison with a new Date() it would be very nice if that new Date() is also in the current time of the Europe/Amsterdam timezone.
After searching the internet for solutions, I started to get the feeling that I will have to update every place in my app where a Date is used and force the use of the target timezone, like the solution of this stackoverflow post: Converting UTC dates to other timezones
Does anybody know how to get this done in a more easy and cleaner way?

The answer for anyone using java.time, the modern Java date and time API.
java.time does not include an option for setting the JVM default time zone. And wisely so. It’s not something you should want to do. By doing it you affect every program running in the same JVM, and also every part of your program and other program in the JVM may set it differently, ruining your intentions.
Avoid the need
In your time operations be explicit about which time zone you want, and you will always know what you get independently of the JVM setting. Example:
System.out.println(ZonedDateTime.now(ZoneId.of("Asia/Dushanbe")));
Example output:
2021-05-09T00:36:25.171213+05:00[Asia/Dushanbe]
System.setProperty
If you have already written a lot of code relying on the default time zone of the JVM, the hack to set it is:
System.setProperty("user.timezone", "Australia/Tasmania");
System.out.println(ZonedDateTime.now());
This just printed:
2021-05-09T05:38:03.568350+10:00[Australia/Tasmania]
It’s not very robust, though, for the reasons mentioned in the beginning.
If you want validation of the string you are passing, use:
System.setProperty("user.timezone", ZoneId.of("Australia/Tasmania").getId());
Disclaimer
It seems from your question that you are already using the old, poorly designed and long outdated java.util.Date class and friends. I still wanted to post the answer for users who have the option to go modern. (You may also use each of the two ideas presented with the out-dated API.)

I would try TimeZone.setDefault(TimeZone) like in:
TimeZone.setDefault(TimeZone.getTimeZone("Europe/Amsterdam"));
(advice to check returned time zone, getTimeZone does not throw exception for unknown time zone - or use ZoneId instead of the String)
see TimeZone (this also mentions the user.timezone system property)
but I am not an android programmer/user

How to create a Calendar or Day Picker in AnyLogic?

I want to create a UI to enable the user to set specific configuration from date1 to date2. To pick the dates I want the calendar to pop up.
To create the calendar, I can put a combo box to choose the month, along with 30 buttons as representative of the days (enabling/disabling some days based on the selected month).
If you know a better way to do this, I would be very thankful if you share it with me!
Thanks!

Please don't do it like that. You have the full power of Java at your hands. There are many Java libraries for date pickers like this one:
https://toedter.com/jcalendar/
Download the .jar file and add it to your model dependencies. Then you can create the date picker object using the respective API.
Here is an AnyLogic example model but it currently only works in AnyLogic 7 (since 8 massively changed how things are displayed). To get it working in AL 8, you might need to ask AL support.

Timezone selection in tapestry date picker as per clients Timezone

We are using Tapestry at our Presentation Tier.
Now we have following situation :
Our application is going to be used by different users from different countries, with different timezones.
In our application we are using Tapestry 5.2.x datepicker,
so how can we allow date selection as per user's Timezone?

https://issues.apache.org/jira/browse/TAP5-841
the patch as per my comment does work. ie working with a string (2012-01-01) instead of a number (millis since epoch).

Java. Creating a date according to user timezone in web-application

I have a Java web-application. In some places there is showing of dates. I want to display the date according to user timezone. How do I create a date for the show if I know the timezone the user?
I use the class Date.
Any help would be greatly appreciated.

There is no standard way to do this. On client side, you may capture time using Java script and send that information to the server. On server side, you can covert the time to Coordinated Universal Time.

Use GregorianCalendar rather than Date when you have any requirements besides basic display. You can create one for the current time by time-zone using new GregorianCalendar(TimeZone).

Client side time zone support in GWT

I'm working on a GWT app where I need to support the following scenario:
The server is located in time zone A
The client's browser is set to time zone B
The GWT app is configured to display date/time in time zone C
Since GWT does not support Calendar and the native support for time
zones in javascript is non-existent I can't think of a nice and clean
solution to this problem.
Have any of you done something similar or do you know of any good
utils I could use?
Thanks!

In my experience, the following best practice significantly reduces complexity and confusion when dealing with dates and timezones in gwt:
Whenever operating/storing dates within the application, treat all dates as milliseconds since epoch in GMT timezone. You can store them as string or int, it doesn't really make a difference.
Whenever displaying the date to the end user, format the date using appropriate timezone.
For your case, when you create a date on the Server (timezone A) convert it to milliseconds since epoch in GMT before sending it to the Client. On the client, use DateTimeFormat (or write your own date formatter util) to convert it into either timezone B or timezone C as appropriate.

You can't change the GWT timezone, hence all java.util.Date's has the browser timezone. You will need to handle the current timezone setting manually.
I see 3 options:
You manage the timezone conversion yourself.
You override the serializer/deserializer of java.util.Date like in this post. And maybe using a custom java.util.Date implemtation, that overrides the getTimezoneOffset(). This approach requires recompilation of the GWT API!.
You implement your own Date, either by extending java.util.Date (like in option 2) or wrapping it with some timezone object. In this option CustomFieldSerializer's may still be usefull, but there is no need for recompiling the GWT API.
I would prefer option 3. At least until GWT RPC maybe someday will support for overriding the CustomFieldSerializer's
Usefull date/time formatting hints.

Dave Paroulek's answer is the right approach. If you want to see an example of this, we created widgets that work independent of TimeZone and process the values on the server-side where we have all of the TimeZone information we need.
UTCDateBox - Wrapper around the GWT DateBox and always chooses the date at midnight in GMT and represents the value as a Long instead of a Date.
UTCTimeBox - New widget that always chooses a time as millis since midnight, independent of timezone, also represented as a Long.
UTCDateTimeUtils - Server-side code that splits a Date into 2 Long values appropriate for UTCDateBox and UTCTimeBox in a given TimeZone and combines them back into a Date in a given TimeZone.
Here is an example of the date the time controls being used together.
Blog article describing their implementation.
These widgets are available on GitHub.

I'm assuming you are using RPC calls for server-client communication here. Also assuming that you don't care about timezone B, and you know what timezone C is on the server.
You have a few options here:
Calculate the desired date in the server (no Java limits on what you can do there) and send it in a String to be displayed to the client, so you don't have to do anymore transformations on the client.
or:
Calculate the offset between timezone A and C on the server, apply it to all the Date objects you are passing to the client and just display them on the client.
if for some reason none of these were valid for you
Calculate the offset, send it to the client and apply it to any Date you receive from the server by transforming to ms, adding the offset and then creating a Date object again.

see this demo project
GWT timezone demo project

I created a GWT-compatible Java version of the jsTimezoneDetect Javascript library specifically for this purpose. This should provide a (very good guess of) the timezone name purely on the client side. Feel free to try it out and let me know if it works or doesn't work for you.