Java - 从不同于本地的时区转换为 UTC [英] Java - Convert From A Timezone Different From Local To UTC

问题描述

所以从我读到的关于这个问题的所有帖子(例如，将时间戳转换为 UTC 时区).

So from all the posts I read about this issue (for example, Convert timestamp to UTC timezone).

我了解到进行这种转换的方法是:

I learn that a way to do this conversion is :

SimpleDateFormat dfmaputo = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss a");
dfmaputo.setTimeZone(TimeZone.getTimeZone("UTC"));
long unixtime = dfmaputo.parse(data.get(1)).getTime();
unixtime = unixtime / 1000;

output: 
original date (Maputo Timezone) -- 11/5/2015 1:39:45 PM
unix timestamp in UTC --- 1446687585
data.get(1) is the string with the maputo datetime.

我不明白为什么我没有得到 UTC 值.当我转换 unix 时间戳时，我希望在 UTC 中使用 Maputo 时区.

I don't understand why I'm not getting the UTC value. When I convert the unix timestamp, that I was expecting to be in UTC, I get the original datetime with Maputo Timezone.

我错过了什么吗?

我是否需要先转换为本地时区，然后再转换为 UTC?

Do I need to convert first to my local timezone and than to UTC?

解决方案

Calendar maputoDateTime = Calendar.getInstance(TimeZone.getTimeZone("Africa/Maputo"));
maputoDateTime.setTimeZone(TimeZone.getTimeZone("GMT"));
Long unixtimeGMT = maputoDateTime.getTimeInMillis() / 1000;

我应该使用日历而不是 SimpleDateFormat.

Instead of SimpleDateFormat I should use Calendar.

首先，我需要设置输入日期的时区 (Africa/Maputo)，然后将其设置为我需要的时区 (GMT).只有这样我才能得到正确的 Unix 时间戳.

First I needed to set the input date's timezone (Africa/Maputo) and then set it to the one I needed (GMT). And only then I could get the correct unix timestamp.

感谢@BastiM 在如何更改java.util.Calendar/Date 的 TIMEZONE

Thanks to @BastiM reply in How to change TIMEZONE for a java.util.Calendar/Date

感谢您的回复和帮助.

推荐答案

如果在字符串末尾添加 CAT 时区标识符并且格式化程序掩码有 z 字母怎么办?如果那是你总是得到的，而源数据没有给出时区值.

What if you add CAT timezone identifier to the end of string and formatter mask has z letter? If thats what you always get and source data does not give timezone value.

    String sdt = "11/5/2015 11:39:45 PM CAT";
    SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy hh:mm:ss a z", Locale.US);
    Date dt = sdf.parse(sdt);
    Calendar cal = Calendar.getInstance();
    cal.setTimeInMillis(dt.getTime());
    System.out.println(dt + ", utc=" + dt.getTime());
    System.out.println(cal);

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