以ISO 8601格式生成昨天的日期时间字符串

Question

In Android, I am using java.util.Calendar to get yesterday date in format yyyy-MM-dd'T'hh:mm:ss'Z' . For example if today is 31 May 2017 , I want to get yesterday date as 2017-05-30T00:00:00Z .

Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);

// Date End = yesterday
calendar.add(Calendar.DATE, -1);
Date dateEnd = calendar.getTime();

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ss'Z'");
this.dateEndStr = formatter.format(dateEnd);

I am expecting output to be 2017-05-30T00:00:00Z . But it gives me 2017-05-30T12:00:00Z .

What is wrong here? Is it TimeZone related? My timezone is GMT/UTC + 08:00 hour .

Answer 1

I am probably answering more than you asked about, but now you have asked, why not take all of it? I see two or three things wrong with your code. bowmore in another answer has touched on all three already, but I think a couple of them can be made a little clearer still:

1. Yes, the first and most serious is time zone related. You need to decide, and you should make explicit in your code, which time zone you use. And it is incorrect to give Z as time zone on a time at zone offset +08:00. Z is for Zulu time, another name for UTC.
2. In your format pattern, you should use capital HH for hour of day.
3. You should prefer the modern date and time classes, in principle always, but at least for a case like this. With these you won't be able to make the errors you did with the oldfashioned SimpleDateFormat .

Time zone

Time zone is crucial to your task. You need to decide whether yesterday's date in UTC (as your requested output says), in GMT/UTC + 08:00 or, say, in the the JVM's current time zone (which can be changed at any time while your program is running). This snippet uses UTC:

this.yesterdayAtStartOfDay = LocalDate.now(ZoneOffset.UTC)
    .minusDays(1)
    .atStartOfDay(ZoneOffset.UTC)
    .format(DateTimeFormatter.ISO_INSTANT);

Running it just now, the result is

2017-05-30T00:00:00Z

Instead of the last line of the snippet, you may use the even simpler:

    .toString();

This gives the same result because atStartOfDay() gives a ZonedDateTime with time zone Z, and its toString() method gives the ISO 8601 string you requested.

If you want yesterday's date in another time zone, in the first line of the snippet (and only the first line) use for example ZoneOffset.ofHours(8) , ZoneId.of("Asia/Hong_Kong") or ZoneId.systemDefault() as time zone.

In case you are using Java 6 or 7, to use the modern date and time classes you need to get the ThreeTen-Backport library . Even though I always think twice before introducing an external dependency, I recommend this one warmly for your task.

That back-port is further adapted for Android in the ThreeTenABP project.

Answer 2

This is simply a mistake in your SimpleDateFormat .

You use hh for the hours, but that shows the one based hour of AM/PM.

These are the relevant symbols (from SimpleDateFormat )

• H Hour in day (0-23)
• k Hour in day (1-24)
• K Hour in am/pm (0-11)
• h Hour in am/pm (1-12)

You want this : "yyyy-MM-dd'T'HH:mm:ss'Z'"

Remark

You say your timezone is +08:00 hours, and you calculate the date using that timezone, however you format it as if it's in the Zulu timezone (which has an offset of +00:00)

BONUS

In Java 8 all this Calendar manipulating goes away :

ZonedDateTime yesterday = ZonedDateTime.now().with(ChronoField.NANO_OF_DAY, 0).minusDays(1);
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss'Z'");
dateEndStr = formatter.format(dateEnd);

Answer 3

To subtract days, you should use:

Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_MONTH, -5);

Answer 4

cal.add(Calendar.DAY_OF_YEAR, -1);