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Calculate time from date taken with different timezone

I have a MySQL database which is storing a datetime value, let's say 2020-10-11 12:00:00. (yyyy-mm-dd hh:mm:ss format)
The type of this date (in mysql) is DATETIME
When I retrieve this data in my controller, it has the java 7 type "Date". But it adds a timezone CEST due to my locale I suspect. Here I already find confusing that when displaying this date which is not supposed to have a timezone attached it actually has... and the debugger says it is "2020-10-11 12:00:00 CEST".
My problem is that date was not stored with the CEST timezone. It was stored with the America/New_York one, for example. EDIT: What I mean with this line, is that the date was stored from new york using the timezone of new york. So, it was really 12:00:00 AM there, but here in Madrid it was 18:00:00 PM. I need that 18:00:00.
So in New York, someone did an insert at that time. Which means that the time in Europe was different. I need to calculate which time was in Europe when in America was 12AM. But my computer keeps setting that date to CEST when I retrieve it so all my parsing attempts are failing... This was my idea:
Date testingDate // This date is initialized fetching the "2020-10-11 12:00:00" from mySql
Calendar calendar = new GregorianCalendar()
calendar.setTime(testingDate)
calendar.setTimeZone(TimeZone.getTimeZone("America/New_York")
SimpleDateFormat localDateFormatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
TimeZone localTimeZone = TimeZone.getTimeZone("Europe/Madrid")
localDateFormatter.setTimeZone(localTimeZone)
String localStringDate = localDateFormatter.format(calendar.getTime())
Date newDate = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(localStringDate)
Here my idea is that: I create a brand new calendar, put on it the time that I had on America and I also say hey this calendar should have the America Timezone. So when I get the time of it using a formatter from Europe it should add the corresponding hours to it. It makes a lot of sense in my head but it is just not working in the code D: And I really don't want to calculate the time difference by myself and adding or substracting the hours because that would look extremely hardcoded in my opinion.
Can any one give me some ideas of what I'm interpreting wrong or how should I tackle this problem in a better way?
Important: I'm using java 7 and grails 2.3.6.

My problem is that date was not stored with the CEST timezone. It was stored with the America/New_York one, for example.
From what I know of MySQL, this is impossible.
Calendar calendar = new GregorianCalendar()
No, don't. The calendar API is a disaster. Use java.time, the only time API in java that actually works and isn't completely broken / very badly designed. If you can't (java 7 is extremely out of date and insecure, you must upgrade!), there's the jsr310 backport. Add that dependency and use it.
Let me try to explain how to understand time first, because otherwise, any answer to this question cannot be properly understood:
About time!
There are 3 completely different concepts and they are all often simplified to mean 'time', but you shouldn't simplify them - these 3 different ideas are not really related and if you ever confuse one for another, problems always occur. You cannot convert between these 3 concepts unless you do so deliberately!
"solarflares time": These describe the moment in time as a universal global concept that something occurred or will occur. "That solar flare was observed at X" is a 'solarflares' time. Best way to store this is millis since epoch.
"appointment time": These describe a specific moment in time as it was or will be in some localized place, but stated in a globally understandable way. "We have a zoom meeting next tuesday at 5" is one of these. It's not actually constant, because locales can decide to adopt new timezones or e.g. move the 'switch date' for daylight savings. For example, if you have an appointment at your dentist on 'november 5th, at 17:00, 2021', and you want to know how many hours are left until your appointment starts, then the value should not change just because you flew to another timezone and are looking at this number from there. However, it should change if the country where you made the appointment in decided to abolish daylight savings time. That's the difference between this one and the 'solarflares' one. This one can still change due to political decisions.
"wake-up-alarm time": These describe a more mutable concept: Some way humans refer to time which doesn't refer to any specific instant or is even trying to. Think "I like to wake up at 8", and thus the amount of time until your alarm will go off next is continually in flux if you are travelling across timezones.
Now, on to your question:
I have a MySQL database which is storing a datetime value, let's say 2020-10-11 12:00:00. (yyyy-mm-dd hh:mm:ss format)
Not so fast. What exact type does that column have? What is in your CREATE TABLE statement? The key thing to figure out here is what is actually stored on disk? Is it solarflare, appointment, or wakeup-alarm? There's DATE, DATETIME and TIMESTAMP, and over the years, mysql has significantly changed how these things are stored.
I believe that, assuming you are using the modern takes on storage (So, newish mysql and no settings to explicitly emulate old behaviour), e.g. a DATETIME stores sign, year, day, hour, minute, and second under the hood, which means it is wakeup alarm style: There is no timezone info in this, therefore, the actual moment in time is not set at all and depends on who is asking.
Contrast to TIMEZONE which is stored as UTC epoch seconds, so it's solarflares time, and it doesn't include any timezone at all. You'd have to store that separately. As far as I know, the most useful of the 3 time representations (appointment time) is not a thing in mysql. That's very annoying; mysql tends to be, so perhaps par for the course.
In java, all 3 concepts exist:
solarflares time is java.time.Instant. java.util.Date, java.sql.Timestamp, System.currentTimeMillis() are also solarflares time. That 'Date' is solarflares timestamp is insane, but then there is a reason that API was replaced.
appointment time is java.time.ZonedDateTime
wakeup-alarm time is java.time.LocalDateTime.
When I retrieve this data in my controller, it has the java 7 type "Date".
Right. So, solarflares time.
Here's the crucial thing:
If the type of time stored in MySQL does not match the type of time on the java side, pain happens.
It sure sounds like you have wakeup-alarm time on disk, and it ends up on java side as solarflares time. That means somebody involved a timezone conversion. Could have happened internally in mysql, could have happened in flight between mysql and the jdbc driver (mysql puts it 'on the wire' converted), or the jdbc driver did it to match java.sql.Timestamp.
The best solution is not to convert at all, and the only real way to do that is to either change your mysql table def to match java's, so, make that CREATE TABLE (foo TIMESTAMP), as TIMESTAMP is also solarflares time, or, to use, at the JDBC level, not:
someResultSet.getTimestamp(col);
as that returns solarflares time, but:
someResultSet.getObject(col, LocalDateTime.class);
The problem is: Your JDBC driver may not support this. If it doesn't, it's a crappy JDBC driver, but that happens sometimes.
This is still the superior plan - plan A. So do not proceed to the crappy plan B alternative unless there is no other way.
Plan B:
Acknowledge that conversion happens and that this is extremely annoying and errorprone. So, make sure you manage it, carefully and explicitly: Make sure the proper SET call is set up so that mysql's sense of which timezone we are at matched. Consider adding storing the timezone as a column in your table if you really need appointment time. etcetera.

Thanks to #rzwitserloot I was able to find out a solution.
First I'll get the data from the database. I'll get rid of any timezone added by the driver / mysql by converting it to a LocalDateTime. Then, I'll create a new ZonedDateTime using the Timezone that was used when storing the data in the database.
Once I have a ZonedDateTime, it is time to convert it using my current timezone. I'll get a new ZonedDateTime object with the proper time.
Then I just add a few more lines to convert it back to my main "Date" class:
I've used the ThreeTen backport as suggested.
Date dateMySQL //Initialized with the date from mysql
Calendar calendar = new GregorianCalendar()
calendar.setTime(dateMySQL)
org.threeten.bp.LocalDateTime localDateTime = org.threeten.bp.LocalDateTime.of(calendar.get(Calendar.YEAR), calendar.get(Calendar.MONTH)+1,
calendar.get(Calendar.DAY_OF_MONTH), calendar.get(Calendar.HOUR_OF_DAY), calendar.get(Calendar.MINUTE),
calendar.get(Calendar.SECOND))
String timezone //Initialized with the timezone from mysql (Ex: "America/New_York")
ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, ZoneId.of(timezone))
ZonedDateTime utcDate = zonedDateTime.withZoneSameInstant(ZoneId.of("Europe/Madrid"))
calendar.setTimeInMillis(utcDate.toInstant().toEpochMilli())
Date desiredDate = calendar.time
dateMySQL: "2020-10-11 10:00:00" // CEST due to my driver
timezone: "America/New_York"
desiredDate: "2020-10-11 19:00:00" // CEST Yay!

Create calendar from String date without knowing format

I want to create an instance of Calendar with string date coming from server. Now I don't know what format server is sending .
It can be changed for different countries. I know I can ask them to add another key for dateFormat and create Calendar from it. But still I want to know Is there any way to create Calendar Instance without knowing current string date format.
I have gone through this and this. But none fulfill my requirement

This is impossible.
If the server sends the value "1/2/2017", you have no way of knowing if this refers to January 2nd or February 1st.
If the server sends the value "מָחָר", in theory you could realize that this might be a Hebrew translation of the word "tomorrow" (at least, according to Google Translate), but even then, it is not clear whether this is to be taken relative to today or some other date.
If the server sends the value "I want to create an instance of Calendar with string date coming from server", you have no means of creating a date from that, at least using any algorithm that would make sense to people.
And so on.
The only reason a server should return a date in an arbitrary format is if the date would only ever be read by the user who provided the value in the first place and presented as plain text verbatim, without parsing. Otherwise, the server should supply the date in a standardized format, with the UI consuming that date being responsible for formatting it in a user-friendly (and, ideally, locale-aware) fashion.
You're welcome to try to brute-force the problem, iterating over a series of date formats and seeing if any result in a seemingly-valid date. This fails the 1/2/2017 scenario (as there are at least two formats that would return a seemingly-valid date), but perhaps you know enough about the server to narrow down the possible formats to reduce the odds of collisions like this.

The Joda Date & Time API has a date parser which can parse date strings in many formats. Note that some datetime strings can be ambiguous: 10-09-2003 could mean October 9 or September 10.

Best way to store time in java, in format of HH:MM

After doing my research I wasn't able to find a method or data type that should be used for variable in order to store time in format of HH:MM, I did find methods to get this from a string like "14:15:10", but I think this is not the best way, as I'll need to add or subtract from time. I tried doing this as a double, but ran into following issue, when you have a time like 05.45 stored and add 0.15 (or 15 minutes) to it, the result is 05.60 where as with HH:MM format you'd expect it to be 06.00.
I'm looked through java documentation and still am, but can't seem to find any way to achieve this, closest I got to is date format like dd/mm/yyyy hh:mm:ss

Use Joda Time. It provides much better operations to do date/time manipulation than standard java dates. If you want to use internal JDK classes, use java.util.Date.

Since Java 8, you can use the new API for dates and times, including Instant, ZonedDateTime and LocalDateTime. This removes the use for the third party library Joda time. It also makes calculations more easy and correct. The advice below is a bit dated but still has some good points.
—————
What you definitely should NOT do is store them in your own custom format. Store the Long value that represents the Unix Epoch.
A DateTime is nothing more than a number to a computer. This number represents the amount of seconds (or milliseconds) since 1970-01-01 00:00:00 UTC. It's beyond the scope of this answer to explain why this date was universally chosen but you can find this by searching for Unix Epoch or reading http://en.wikipedia.org/wiki/Unix_time.
This also means there is NO timezone information stored in a DateTime itself. It is important to keep this in mind when reasoning about dates and times. For things such as comparing DateTime objects, nothing concerning localization or timezones is done. Only when formatting time, which means as much as making it readable to humans, or for operations such as getting the beginning of the day, timezones come into play.
This is also why you shouldn't store the time like 20:11:15 in a string-like format because this information is meaningless without timezone information. I will give you 1 example here: Consider the moment when the clock is moved back 1 hour, such as when moving away from daylight savings time. It just happened in a lot of countries. What does your string 02:30 represent? The first or the second one?
Calculations such as subtraction are as easy as doing the same with numbers. For example: Date newDate = new Date(date1.getTime() - date2.getTime());. Or want to add an hour to a date? Date newDate = new Date(oldDate.getTime() + 1000 * 60 * 60);
If you need more complex stuff then using Joda time would be a good idea, as was already suggested. But it's perfectly possible to just do even that with the native libraries too.
If there's one resource that taught me a lot about date/time, it would be http://www.odi.ch/prog/design/datetime.php

Java has java.sql.Time format to work with time-of-day values. Just import it and create variables.
import java.sql.Time;
//now we can make time variables
Time myTime;
Just saw it on https://db.apache.org/derby/docs/10.4/ref/rrefsqlj21908.html

The answer that is right for your case depends on what you want to do.
Are you using a RDBMS as your persistence engine?
If so, are you already working with legacy data formats or are you building a database from the ground up?
Are you simply storing this data, or will you be doing extensive date arithmetic and/or precedence calculations?
Are you in one time zone or do you need to work with time instants across many time zones?
All of these things are important and factor into your decision of how to represent your times and dates.
If your needs require a lot of date arithmetic (eg. determining days between dates) or sorting based on timestamps, then consider using a floating point date format. The advantage of using a numeric format for timestamps is that doing date arithmetic and comparison/sorting operations becomes trivial; you merely do simple arithmetic. Another advantage is that floats and longs are primitive data types. They do not need to be serialized, they are already extremely lightweight, and everything you need to use them requires no external dependencies.
The main disadvantage to using numeric formats for timestamps is that they are not human friendly. You'll need to convert them to and from a String format to allow users to interact. Oftentimes, this is worth the effort. See: How do I use Julian Day Numbers with the Java Calendar API?
I recommend that you consider storing timestamps as Julian Day Numbers (JDNs) or Modified Julian Day Numbers (MJDs). Both will represent dates and times to millisecond precision using an 8 byte float. Algorithms for converting to and from display formats for both of these are highly standardized. They confer all the advantages of using numeric dates. Moreover, they are defined only for GMT/UTC which means that your timestamps are already universalizable across time zones right out of the box (as long as you localize properly).

If you dont want the full date object, your best bet is to store it in a string, but I personally would still recommend date as it also contains a lot of convenient methods that will come in handy. You can just get the time as a whole from a date object and ignore the rest.

In terms of "storing" a date, you should use a long. This is how the system sees it and how all calculations are performed. Yes, as some point out you will eventually need to create a String so a human can read it, but where people run into trouble is when they start thinking of a date in terms of format. Format is for readability, not for calculations. java.util.Date and java.util.Calendar are fraught with issues (Effective Java, Bloch, et. al. has plenty to say about it) but are still the norm if you need handy date operations.

Designing SQLite Database without Date format

I am trying to store Date in sqlite database in android mm/dd/yyyy , I have read about this What happens if I don't have HH:mm:ss (which is not needed for my application)?.
Is avoiding the date format of database by splitting it up a method used in the industry? ex:
Db Columns:
_id INTEGER;
data TEXT;
day INTEGER;
month INTEGER;
year INTEGER;
Thanks for any advice.

You could store the Date in milliseconds. Then build the appropriate format according the user's Locale at run time.

If you wand to use ony little memory, consider to store ddMMyyyy in a single intvariable.
either easy to read in logoutput
day+month*100+year*10000
day, month year can be extracted by devision and modulo operations
or for faster to handle for the computer
day&0x1F+(month<<5)&0xF+(year<<9)
day, month year can be extracted by >> and & and operations

For compatibility reasons, my app stores dates as strings, in ISO "YYYY-MM-DD" format. This is a special case though. For most purposes, Sam is the man.

SQLite's date functions also understand date strings in yyyy-mm-dd format.
Splitting up dates into their three fields is possible, but then you will not be able to easily do computations on them.

Store time in 12-hour format in SQLite and some calculations on it in Android

I am working on some time stuff on Android. I get the time from a TimePicker and I want to store it in the database. I want to store time in a 12-hour format.
I also need to do some calculations on that time. If I store it as a String, it will involve a lot of coding to do calculations on that String.
Is there a better solution?
Any help would be appreciated. Thanks in advance.

In my androidapp i am storing datetime as long in java and database.
(Unix Time, number of seconds since 1970-01-01. as #hawaii.five-0 stated)
These numbers can easily be compared for earlier/later dates but are hard to read for a human when looking into raw database.
For displaying dates these long numbers are converted back to java Date with new Date(longValue).

Best practice is to store time as Unix timestamps:
Easy calculations (you can just add and subtract time in seconds, so to add an hour you add 60*60 = 3600 seconds)
Easy formatting: the Java Date and DateFormat classes can easily handle Unix timestamps.
Example of 12 hour formatting:
public String getDateFormatted(Date datum) {
DateFormat dfm = new SimpleDateFormat("MM/dd/yyyy K:m a"); //example: 05/23/2012 11:34 PM
return dfm.format(datum);
}

Unfortunately sqlite does not support Date format... Take a look at sqlite affinity.
So you have to store it in text or numeric format.. And apply your computations after retriving it from the database..