Java日历中的未来日期给出了一种奇怪的行为

Question

I have an application which I create dates that a user can select to an appointment. If a user start to work at 9, and an appointment takes 2 hours, I create dates at 9, 11, 13... until a limit, of course. And then I change the day and start again. This is the code for doing this:

    public List<Agenda> createListOfDates(Calendar initial, Calendar end, 
        int appointmentDuration, int lunchTimeDuration, int lunchTimeStart) {

        List<Agenda> agendaList = new ArrayList<Agenda>();

        Agenda agenda = new Agenda();
        agenda.setWorkingHour(initial.getTime());
        agendaList.add(agenda);
        while (true) {

            initial.add(Calendar.HOUR_OF_DAY, appointmentDuration);
//          Logger.error("" + initial.getTime());

            if (initial.getTime().after(end.getTime())) {
                break;

            } else if (initial.get(Calendar.HOUR_OF_DAY) == lunchTimeStart
                    && initial.get(Calendar.DAY_OF_WEEK) != Calendar.SATURDAY
                    ) {
                initial.add(Calendar.HOUR_OF_DAY, lunchTimeDuration);
                agenda = new Agenda();
                agenda.setWorkingHour(initial.getTime());
                agendaList.add(agenda);

            } else {
                agenda = new Agenda();
                agenda.setWorkingHour(initial.getTime());
                agendaList.add(agenda);
            }
        }

        for(Agenda agendaX : agendaList){
        Logger.info("" + agendaX.getWorkingHour());

}

        return agendaList;
    }

I am working with the "America/Sao_Paulo" timezone to create these dates. I set the variables "initial" and "end" as "America/Sao_Paulo". My system timezone is "GMT", and that is ok, because I want to save these dates in GMT in the database. When I print the dates in last "for", magically it is already converted from "America/Sao_Paulo" to "GMT" and it is printing right. The strange thing is that from a certain date, it changes the time zone. Example of prints:

Sat Mar 30 12:00:00 GMT 2019
Sat Mar 30 14:00:00 GMT 2019
Sat Mar 30 16:00:00 GMT 2019
Sat Mar 30 18:00:00 GMT 2019
Mon Apr 01 13:00:00 BST 2019
Mon Apr 01 15:00:00 BST 2019
Mon Apr 01 18:00:00 BST 2019
Mon Apr 01 20:00:00 BST 2019
Mon Apr 01 22:00:00 BST 2019

While is in GMT, it is right, but I can't understand this BST. Can it be because it's too much in the future? It always starts on April.

Answer 1

Your system time isn't GMT, it's Europe/London (or something similar). In March London time coincides with GMT. Not in April. That's why.

getWorkingHour() returns an instance of Date (another poorly designed and long outdated class, but let that be a different story for now). When you append it to the empty string, Date.toString is implicitly called and builds the string using your system time zone. During standard time it prints GMT as time zone abbreviation. Summer time (DST) begins in London on the last Sunday of March, in this case March 31. So in April Date.toString on your JVM uses British Summer Time and its abbreviation, BST for printing the time.

The good solution involves two changes:

1. Don't rely on the JVM's default time zone. It can be changed at any time from another part of your program or another program running in the same JVM, so is too fragile. Instead give explicit time zone to your date-time operations.
2. Skip the old date-time classes Calendar and Date and instead use java.time, the modern Java date and time API. It is so much nicer to work with and gives much clearer code, not least when it comes to conversions between time zones.

Instead of Calendar use ZonedDateTime . Depending on the capabilities of your JDBC driver, convert it to either Instant or OffsetDateTime in UTC for saving to the database.

To create a ZonedDateTime , one option is to use one of its of methods (there are several):

    ZonedDateTime initial = ZonedDateTime.of(2019, 3, 10, 9, 0, 0, 0, ZoneId.of("America/Sao_Paulo"));

This creates a date-time of March 10, 2019 at 09:00 in São Paolo. To add 2 hours to it:

    int appointmentDuration = 2;
    ZonedDateTime current = initial.plusHours(appointmentDuration);
    System.out.println(current);

Output:

2019-03-10T11:00-03:00[America/Sao_Paulo]

To convert to an Instant for your database:

    Instant inst = current.toInstant();
    System.out.println(inst);

Output:

2019-03-10T14:00:00Z

Instants are time zone neutral, just a point in time, but print in UTC. Some JDBC drivers accept them for UTC times. If yours doesn't happen to, you will need to give it an OffsetDateTime instead. Convert like this:

    OffsetDateTime odt = current.toOffsetDateTime().withOffsetSameInstant(ZoneOffset.UTC);
    System.out.println(odt);

Output:

2019-03-10T14:00Z

Note that I give UTC explicitly rather than relying on the JVM default. So this is explicitly in UTC. You notice that the date and time agree with what was printed from the Instant .