Formatting Dates and Times Overview
Formatting Dates and Times Overview
Date and time formatters are used to convert dates and times from their internal representations to textual form and back again in a language-independent manner. The date and time formatters use UDate, which is the internal representation. Converting from the internal representation (milliseconds since midnight, January 1, 1970) to text is known as "formatting," and converting from text to milliseconds is known as "parsing."
DateFormat
DateFormat helps format and parse dates for any locale. Your code can be completely independent of the locale conventions for months, days of the week, or calendar format.
Formatting Dates
The DateFormat interface in ICU enables you to format a Date in milliseconds into a string representation of the date. It also parses the string back to the internal Date representation in milliseconds.
DateFormat* df = DateFormat::createDateInstance();
UnicodeString myString;
UDate myDateArr[] = { 0.0, 100000000.0, 2000000000.0 };
for (int32_t i = 0; i < 3; ++i) {
myString.remove();
cout << df->format( myDateArr[i], myString ) << endl;
}
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To format a date for a different Locale, specify it in the call to:
DateFormat* df = DateFormat::createDateInstance ( DateFormat::SHORT, Locale::getFrance()); |
Parsing Dates
Use a DateFormat to parse also:
UErrorCode status = ZERO_ERROR; UDate myDate = df->parse(myString, status); |
Producing Normal Date Formats for a Locale
Use createDateInstance to produce the normal date format for that country. There are other static factory methods available. Use createTimeInstance to produce the normal time format for that country. Use createDateTimeInstance to produce a DateFormat that formats both date and time. You can pass different options to these factory methods to control the length of the result; from SHORT to MEDIUM to LONG to FULL. The exact result depends on the locale, but generally:
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SHORT is numeric, such as 12/13/52 or 3:30pm
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MEDIUM is longer, such as Jan. 12, 1952
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LONG is longer, such as January 12, 1952 or 3:30:32pm
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FULL is completely specified, such as Tuesday, April 12, 1952 AD or 3:30:42pm PST
Producing Relative Date Formats for a Locale
Setting Time Zones
You can set the time zone on the format. If you want more control over the format or parsing, cast the DateFormat you get from the factory methods to a SimpleDateFormat. This works for the majority of countries.
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Remember to check getDynamicClassID() before carrying out the cast. |
Working with Positions
You can also use forms of the parse and format methods with ParsePosition and FieldPosition to enable you to:
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Progressively parse through pieces of a string.
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Align any particular field, or find out where it is for selection on the screen.
SimpleDateFormat
SimpleDateFormat is a concrete class used for formatting and parsing dates in a language-independent manner. It allows for formatting, parsing, and normalization. It formats or parses a date or time, which is the standard milliseconds since 24:00 GMT, Jan. 1, 1970.
SimpleDateFormat is the only built-in implementation of DateFormat. It provides a programmable interface that can be used to produce formatted dates and times in a wide variety of formats. The formats include almost all of the most common ones.
Create a date-time formatter using the following methods rather than constructing an instance of SimpleDateFormat. In this way, the program is guaranteed to get an appropriate formatting pattern of the locale.
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DateFormat::getInstance()
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getDateInstance()
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getDateTimeInstance()
If you need a more unusual pattern, construct a SimpleDateFormat directly and give it an appropriate pattern.
Date/Time Format Syntax
A date pattern is a string of characters, where specific strings of characters are replaced with date and time data from a calendar when formatting or used to generate data for a calendar when parsing.
The Date Field Symbol Table below contains the characters used in patterns to show the appropriate formats for a given locale, such as yyyy for the year. Characters may be used multiple times. For example, if y is used for the year, 'yy' might produce '99', whereas 'yyyy' produces '1999'. For most numerical fields, the number of characters specifies the field width. For example, if h is the hour, 'h' might produce '5', but 'hh' produces '05'. For some characters, the count specifies whether an abbreviated or full form should be used, but may have other choices, as given below.
Two single quotes represents a literal single quote, either inside or outside single quotes. Text within single quotes is not interpreted in any way (except for two adjacent single quotes). Otherwise all ASCII letter from a to z and A to Z are reserved as syntax characters, and require quoting if they are to represent literal characters. In addition, certain ASCII punctuation characters may become variable in the future (eg ":" being interpreted as the time separator and '/' as a date separator, and replaced by respective locale-sensitive characters in display).
"Stand Alone" values refer to those designed to stand on their own, as opposed to being with other formatted values. "2nd quarter" would use the stand alone format (QQQQ), whereas "2nd quarter 2007" would use the regular format (qqqq yyyy).
The pattern characters used in the Date Field Symbol Table are defined by CLDR; for more information see CLDR Date Field SymbolTable.
Date Field Symbol Table |
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|---|---|---|---|
| Symbol | Meaning | Example(s) | |
| G | era designator | G, GG, or GGG GGGG GGGGG |
AD Anno Domini A |
| y | year | yy y or yyyy |
96 1996 |
| Y | year of "Week of Year" | Y | 1997 |
| u | extended year | u | 4601 |
| U | cyclic year name, as in Chinese lunar calendar | U | 甲子 |
| Q | quarter | Q or QQ QQQ QQQQ |
02 Q2 2nd quarter |
| q | Stand Alone quarter | q or qq qqq qqqq |
02 Q2 2nd quarter |
| M | month in year | M or MM MMM MMMM MMMMM |
09 Sept September S |
| L | Stand Alone month in year | L or LL LLL LLLL LLLLL |
09 Sept September S |
| w | week of year | w or ww | 27 |
| W | week of month | W | 2 |
| d | day in month | d dd |
2 02 |
| D | day of year | D | 189 |
| F | day of week in month | F | 2 (2nd Wed in July) |
| g | modified julian day | g | 2451334 |
| E | day of week | E, EE, or EEE EEEE EEEEE EEEEEE |
Tues Tuesday T Tu |
| e | local day of week example: if Monday is 1st day, Tuesday is 2nd ) |
e or ee eee eeee eeeee eeeeee |
2 Tues Tuesday T Tu |
| c | Stand Alone local day of week | c or cc ccc cccc ccccc cccccc |
2 Tues Tuesday T Tu |
| a | am/pm marker | a | pm |
| h | hour in am/pm (1~12) | h hh |
7 07 |
| H | hour in day (0~23) | H HH |
0 00 |
| k | hour in day (1~24) | k kk |
24 24 |
| K | hour in am/pm (0~11) | K KK |
0 00 |
| m | minute in hour | m mm |
4 04 |
| s | second in minute | s ss |
5 05 |
| S | millisecond (maximum of 3 significant digits); for S or SS, truncates to the number of letters for SSSS or longer, fills additional places with 0 |
S SS SSS SSSS |
2 23 235 2350 |
| A | milliseconds in day | A | 61201235 |
| z | Time Zone: specific non-location | z, zz, or zzz zzzz |
PDT Pacific Daylight Time |
| Z | Time Zone: ISO8601 basic hms? / RFC 822 Time Zone: long localized GMT (=OOOO) TIme Zone: ISO8601 extended hms? (=XXXXX) |
Z, ZZ, or ZZZ ZZZZ ZZZZZ |
-0800 GMT-08:00 -08:00, -07:52:58, Z |
| O | Time Zone: short localized GMT Time Zone: long localized GMT (=ZZZZ) |
O OOOO |
GMT-8 GMT-08:00 |
| v | Time Zone: generic non-location (falls back first to VVVV) |
v vvvv |
PT Pacific Time or Los Angeles Time |
| V | Time Zone: short time zone ID Time Zone: long time zone ID Time Zone: time zone exemplar city Time Zone: generic location (falls back to OOOO) |
V VV VVV VVVV |
uslax America/Los_Angeles Los Angeles Los Angeles Time |
| X | Time Zone: ISO8601 basic hm?, with Z for 0 Time Zone: ISO8601 basic hm, with Z Time Zone: ISO8601 extended hm, with Z Time Zone: ISO8601 basic hms?, with Z Time Zone: ISO8601 extended hms?, with Z |
X XX XXX XXXX XXXXX |
-08, +0530, Z -0800, Z -08:00, Z -0800, -075258, Z -08:00, -07:52:58, Z |
| z | Time Zone: ISO8601 basic hm?, without Z for 0 Time Zone: ISO8601 basic hm, without Z Time Zone: ISO8601 extended hm, without Z Time Zone: ISO8601 basic hms?, without Z Time Zone: ISO8601 extended hms?, without Z |
x xx xxx xxxx xxxxx |
-08, +0530 -0800 -08:00 -0800, -075258 -08:00, -07:52:58 |
| ' | escape for text | ' | (nothing) |
| ' ' | two single quotes produce one | ' ' | ' |
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Any characters in the pattern that are not in the ranges of ['a'..'z'] and ['A'..'Z'] will be treated as quoted text. For instance, characters like ':', '.', ' ', '#' and '@' will appear in the resulting time text even they are not enclosed within single quotes.The single quote is used to 'escape' letters. Two single quotes in a row, whether inside or outside a quoted sequence, represent a 'real' single quote. |
| A pattern containing any invalid pattern letter results in a failing UErrorCode result during formatting or parsing. |
| Format Pattern | Result |
|---|---|
| yyyy.MM.dd G 'at' HH:mm:ss zzz |
1996.07.10 AD at 15:08:56 PDT |
| EEE, MMM d, ''yy |
Wed, July 10, '96 |
| h:mm a |
12:08 PM |
| hh 'o''clock' a, zzzz |
12 o'clock PM, Pacific Daylight Time |
| K:mm a, z |
0:00 PM, PST |
| yyyyy.MMMM.dd GGG hh:mm aaa |
01996.July.10 AD 12:08 PM |
Time Zone Display Names
ICU supports time zone display names defined by the LDML (Unicode Locale Data Markup Language ) specification. Since ICU 3.8, the vast majority of localized time zone names are no longer associated with individual time zones. Instead, a set of localized time zone names are associated with a metazone and one or more individual time zones are mapped to the same metazone. For example, metazone “America_Pacific” has its own display name data such as “PST” “PDT” “PT” “Pacific Standard Time” “Pacific Daylight Time” “Pacific Time” and these names are shared by multiple individual time zones “America/Los_Angeles”, “America/Vancouver”, “America/Tijuana” and so on. The mapping from individual time zone to metazone is not a simple 1-to-1 mapping, but it changes time to time. For example, time zone “America/Indiana/Tell_City” uses name data from metazone “America_Eastern” until April 2, 2006, but it changes to metazone “America_Central” after the date. So the display name used for “America/Indiana/Tell_City” before the date (e.g. “Eastern Time”) differs from the one after the date (e.g. “Central Time).
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Prior to ICU 3.8, a localized time zone name (except GMT format) and a time zone ID was in 1-to-1 relationship always. Therefore, a time zone name produced by DateFormat can be parsed back to the original time zone. This assumption no longer applies to ICU 3.8 and later releases for all time zone format types. If you program requires to roundtrip specific time zone ID, you must use the generic location format (“VVVV”) explained below. |
There are several different display name types available in the LDML specification.
Time Zone Display Name Types |
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|---|---|---|
| Type | Description | Examples |
| Generic non-location | Reflects wall time, suited for displaying recurring events, meetings or anywhere people do not want to be overly specific. Available in two length options – long and short. | Pacific TimePT |
| Generic partial location | Reflects wall time, used as a fallback format when the generic non-location format is not specific enough. A generic partial location name is constructed from a generic non-location name with a location name. For example, “PT” is shared by multiple time zones via metazone“America_Pacific”. When GMT offset in the time zone at the given time differs from the preferred time zone of the metazone for the locale, location name is appended to generic non-location name to distinguish the time zone from the preferred zone. Available in two length options – long and short. | Pacific Time (Canada)PT (Yellowknife) |
| Generic location | Reflects wall time, suited for populating choice list for time zones. If the time zone is the single time zone available in the region (country), the generic location name is constructed with the region name. Otherwise, the name is constructed from the region name and the city name. Unlike other format types, this name is unique per time zone. | United States (Los Angeles) TimeItaly Time |
| Specific non-location | Reflects a specific standard or daylight time. Available in two length options – long and short. | Pacific Standard TimePDT |
| Localized GMT | A constant, specific offset from GMT in a localized form. | GMT-08:00 |
| RFC822 GMT | A constant, specific offset from GMT in a locale insensitive format. | -0800 |
Each format type in the above table is used as a primary type or a fallback in SimpleDateFormat. The table below explains how ICU time zone format pattern work and its characteristics.
Time Zone Pattern Usage |
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|---|---|---|---|---|
| Pattern | Behavior | Round-trip time at daylight transitions(*) | Round-trip Time Zone | Suggested Usage |
| z, zz, zzz | Short specific non-location format (e.g. “PST”).If the localized data is not available or the short abbreviation is not commonly used for the locale, localized GMT format is used (e.g. GMT-08:00). | yes | no | For displaying a time with a user friendly time zone name. |
| zzzz | Long specific non-location format (e.g. “Pacific Standard Time”).If the localized data is not available, localized GMT format is used (e.g. GMT-08:00). | yes | no | Same as “z”, but longer format. |
| v | Short generic non-location format (e.g. “PT”).If the localized data is not available or the short abbreviation is not commonly used for the locale, generic location format (e.g. “United States(Los Angeles) Time”) is used.If the localized data comes from metazone and the GMT offset at the given time in the specified time zone differs from the preferred time zone of the metazone for the locale, generic partial location format (e.g. “PT (Canada)”) is used. | no | no | For displaying a recurring wall time (e.g. events, meetings) or anywhere people do not want to be overly specific. |
| vvvv | Long generic non-location format (e.g. “Pacific Time”).If the localized data is not available, generic location format (e.g. “United States(Los Angeles) Time”) is used. | no | no | Same as “v”, but longer format. |
| V | Same as “z”, except using the short abbreviation even it is not commonly used for the locale. | yes | no | Same as “z”. |
| VVVV | Generic location format (e.g. “United States (Los Angeles) Time”). | no | yes | For populating a choice list for time zones, because it supports 1-to-1 name/zone ID mapping and is more uniform than other text formats.Also, this is only the pattern supporting time zone round-trip. If your program requires to preserve the original time zone information, use this pattern. |
| Z, ZZ, ZZZ | Localized GMT format (e.g. “GMT-08:00”). | yes | no | For displaying a time in UI in a uniformed manner. |
| ZZZZ | RFC822 GMT format (e.g. “-0800”). | yes | no | For formatting a time for non-user-facing data. |
* At a transition from daylight saving time to standard time, there is a wall time interval occurs twice.
DateFormatSymbols
DateFormatSymbols is a public class for encapsulating localizable date-time formatting data, including time zone data. DateFormatSymbols is used by DateFormat and SimpleDateFormat.
DateFormatSymbols specifies the exact character strings to use for various parts of a date or time For example, the names of the months and days of the week, the strings for AM and PM and the day of the week considered to be the first day of the week (used in drawing calendar grids) are controlled by DateFormatSymbols.
Create a date-time formatter using the createTimeInstance, createDateInstance, or createDateTimeInstance methods in DateFormat. Each of these methods can return a date/time formatter initialized with a default format pattern, along with the date-time formatting data for a given or default locale. After a formatter is created, modify the format pattern using applyPattern.
If you want to create a date-time formatter with a particular format pattern and locale, use one of the SimpleDateFormat constructors:
UnicodeString aPattern("GyyyyMMddHHmmssSSZ", "");
new SimpleDateFormat(aPattern, new DateFormatSymbols(Locale::getUS())
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This loads the appropriate date-time formatting data from the locale.s
Programming Examples
See date and time formatting examples .