How to translate system date from java backend service to the logged in user's time zone?

Here is my issue:
I am building a web based application. Am using RESTful services using java in the backend. These services are hosted on a server in United Kingdom.
My application users mostly fall in European locations. Time difference between the 2 locations is creating issue. Let's say my app users are in Germany. Germany is ahead of UK by 1hr. (Assuming we are in summer). Now, to search items from backend and display in UI, I have a time filter to be applied in back end service.
If my user logs in to my application at Germany at 9am local time, how can I make sure backend service also applies time filter based on Germany time and not UK time.

Here is what I tried so far:

In my repository class(UnitRepository.java) I did this:

    @Query(value="select asu.* from Site_Unit su where su.SCHEDULED_DEPARTURE_TIME between :startTime and :endTime", nativeQuery = true )
List<ResultDTO> getUnitsByTime(@Param("startTime") Timestamp startTimeFilter,
                               @Param("endTime") Timestamp endTimeFilter);

In my DaoImpl class I did this:

public List<ResultDTO> getUnitsByTime() {

    Calendar today12Noon = new GregorianCalendar();
    today12Noon.set(Calendar.HOUR_OF_DAY, 12);
    today12Noon.set(Calendar.MINUTE, 0);
    today12Noon.set(Calendar.SECOND, 0);
    today12Noon.set(Calendar.MILLISECOND, 0);
    java.util.Date endTime = today12Noon.getTime(); // end time is till 12pm of the same day

    java.util.Date startTime = new java.util.Date(); // start time is the time when user logs in to the application

    return unitRepository.getUnitsByTime(new Timestamp(startTime.getTime()), new Timestamp(endTime.getTime()));
}

However, 'startTime' picks up system date for UK time whereas I want it to be for Germany time zone.

Any pointers would be appreciated.(There are few more pieces of code which I left out as it was not relevant here)

First of all, I VERY STRONGLY recommend not to use outdated classes Date and Timestamp and switch to classes in the java.time package. For example ZonedDateTime. Using classes from that package you can easily convert from time in GMT to local time. This is a general suggestion. As for your issue, there are several solutions:

1. user can send you time parameters that include time zone in it: for instance, they will send you an instance of ZonedDateTime that has time zone info and not LocalDateTime which does not. In this case, if you use the ZonedDateTime class as a filter in your query everything will work automatically.
2. You can keep user profile in your app, and there you can store user's time zone. In this case even if user sends you time query parameter without time zone, you can adjust the time based on user's time zone taken from user's profile