TimezoneChange.java

public class TimezoneChange { 

    private static final ZoneId ZONE = ZoneId.of("Europe/Madrid");
    private static final DateTimeFormatter TIME_FORMATTER = new DateTimeFormatterBuilder()
                                                                        .appendPattern("yyyy-MM-dd'T'HH:mm:ss")
                                                                        .appendFraction(MILLI_OF_SECOND, 0, 3, true)
                                                                        .toFormatter();
    
    public static void main(String[] args) {
        Instant converted = ZonedDateTime.of(LocalDateTime.parse(startDate, TIME_FORMATTER), ZONE).toInstant();
        System.out.println("after changing time zone: " + converted);
    }
}