关于时间

全网最全！彻底弄透Java处理GMT/UTC日期时间

概念

• HTTP日期时间。所有HTTP日期/时间戳都必须用格林威治标准时间(GMT)表示，没有例外。对于HTTP来说，GMT完全等于UTC(协调世界时)。
• UTC和GMT的联系。UTC是标准时间参照，像GMT（格林威治时间）、ET（美国东部时间）、PST（太平洋时间）、CST（北京时间）等等都是具体的时区时间。GMT能和UTC直接转换，仅仅是因为碰巧GMT是0时区时间，数值上刚好和UTC是相等的（不需要精确到秒的情况下，二者可以视为相等）。
• UTC偏移量。常见的表示形式有：±[hh]:[mm]、±[hh][mm]、±[hh]这三种方式均可。
• 默认时区。很多公司在阿里云、腾讯云、国内外的云主机上部署应用时，全部都是采用运维设置统一时区：中国时区，这种方式来管理的。
• 自己平时开发中前端就是扔个字符串给我，然后我就给格式化为一个Date类型，并没有传入时区参数？这是因为默认情况下你们交互双发就达成了契约：双方均使用的是北京时间（时区），既然是相同时区，所以互通有无不会有任何问题。不信你把你接口给海外用户调试试？
• ZoneId和ZoneOffset.。若你使用ZoneId时区去表示就很方便喽，比如纽约是西五区，你在任何时候获取其当地时间都是能得到正确答案的，因为它内置了对夏令时规则的处理，也就是说啥时候+1啥时候-1时区自己门清，不需要API调用者关心。没有夏令时问题。ZoneId只是一个用于获取底层规则的ID。之所以采用这种方法，是因为规则是由政府定义的，并且经常变化，而ID是稳定的。
• 根据偏移量得到的ZoneId内部并无现成时区规则可用，因此对于有夏令营的国家转换可能出问题，一般不建议这么去做。
• ZoneId并没有提供设置默认时区的方法,设置默认时区方式完全遵照TimeZone的方式即可。
• 夏令时是为节约能源而人为规定地方时间的制度（鼓励人们早睡早起，不要浪费电，夏天日照时间长尽量多用自然资源）。在夏令时开始这天将时钟往前拨拨1个小时，11.1号这天还原回来。（时区+1，相对非夏令时提早干活）。比如夏令时期间带时区解析后的结果：字符串参数偏移量明明是-05，转换为ZonedDateTime后偏移量成为了-04。

• OffsetDateTime和Instant可用于模型的字段类型，因为它们都表示瞬间值并且还不可变，所以适合网络传输或者数据库持久化。ZonedDateTime不适合网络传输/持久化，因为即使同一个ZoneId时区，不同地方获取到瞬时值也有可能不一样。
• ZonedDateTime可简单认为是LocalDateTime和ZoneId的组合。而ZoneOffset是其内置的动态计算出来的一个次要信息，以确保输出一个瞬时值而存在，毕竟在某个瞬间偏移量ZoneOffset肯定是确定的。
• OffsetDateTime无法支持夏令时等规则，但ZonedDateTime可以很好的处理夏令时调整。OffsetDateTime代表一个瞬时值，而ZonedDateTime的值是不稳定的，需要在某个瞬时根据当时的规则计算出来偏移量从而确定实际值。总的来说，OffsetDateTime和ZonedDateTime的区别主要在于ZoneOffset和ZoneId的区别。如果你只是用来传递数据，请使用OffsetDateTime，若你想在特定时区里做时间显示那么请务必使用ZonedDateTime。

日期/时间模式

jdk8 LocalDate等

参考：Java8新特性日期API by 阿牛

new Date(): Sun Jan 02 19:22:20 CST 2022
============Local===============
LocalDateTime.now(): 2022-01-02T19:22:20.281056
LocalDateTime.of: 2021-01-02T17:23:52
localDate.atTime(localTime): 2021-01-02T17:23:59
getDayOfMonth(): 2
=============Instant========================
System.currentTimeMillis(): 1641122540286
Instant.now(): 2022-01-02T11:22:20.292162Z
Instant.now().toEpochMilli(): 1641122540297
LocalDateTime.now() with systemDefaultZone to toEpochMilli: 1641122540298
Instant.ofEpochSecond(120, 100000): 1970-01-01T00:02:00.000100Z
=============Duration=============
Duration.between: 7
Duration.of Days: 5
Duration.of MILLIS: 1000
=============Period================
Period.of(): 6
Period.between():1
==========================增加和减少日期===========================================
TemporalAdjusters.nextOrSame: 2022-01-08
TemporalAdjusters.lastInMonth: 2022-01-29
==========================格式化日期===========================================
format DateTimeFormatter.ISO_DATE_TIME: 2022-01-02T19:22:20.306401
format DateTimeFormatter.ofPattern: 2022-01-02
parse DateTimeFormatter.ofPattern: 2021-01-04T17:39:30
==========================时区ZoneId===========================================
ZoneId.getAvailableZoneIds()[Asia/Aden, America/Cuiaba, Etc/GMT+9, Etc/GMT+8, Africa/Nairobi, America/Marigot, Asia/Aqtau, Pacific/Kwajalein, America/El_Salvador, Asia/Pontianak, Africa/Cairo, Pacific/Pago_Pago, Africa/Mbabane, Asia/Kuching, Pacific/Honolulu, Pacific/Rarotonga, America/Guatemala, Australia/Hobart, Europe/London, America/Belize, America/Panama, Asia/Chungking, America/Managua, America/Indiana/Petersburg, Asia/Yerevan, Europe/Brussels, GMT, Europe/Warsaw, America/Chicago, Asia/Kashgar, Chile/Continental, Pacific/Yap, CET, Etc/GMT-1, Etc/GMT-0, Europe/Jersey, America/Tegucigalpa, Etc/GMT-5, Europe/Istanbul, America/Eirunepe, Etc/GMT-4, America/Miquelon, Etc/GMT-3, Europe/Luxembourg, Etc/GMT-2, Etc/GMT-9, America/Argentina/Catamarca, Etc/GMT-8, Etc/GMT-7, Etc/GMT-6, Europe/Zaporozhye, Canada/Yukon, Canada/Atlantic, Atlantic/St_Helena, Australia/Tasmania, Libya, Europe/Guernsey, America/Grand_Turk, Asia/Samarkand, America/Argentina/Cordoba, Asia/Phnom_Penh, Africa/Kigali, Asia/Almaty, US/Alaska, Asia/Dubai, Europe/Isle_of_Man, America/Araguaina, Cuba, Asia/Novosibirsk, America/Argentina/Salta, Etc/GMT+3, Africa/Tunis, Etc/GMT+2, Etc/GMT+1, Pacific/Fakaofo, Africa/Tripoli, Etc/GMT+0, Israel, Africa/Banjul, Etc/GMT+7, Indian/Comoro, Etc/GMT+6, Etc/GMT+5, Etc/GMT+4, Pacific/Port_Moresby, US/Arizona, Antarctica/Syowa, Indian/Reunion, Pacific/Palau, Europe/Kaliningrad, America/Montevideo, Africa/Windhoek, Asia/Karachi, Africa/Mogadishu, Australia/Perth, Brazil/East, Etc/GMT, Asia/Chita, Pacific/Easter, Antarctica/Davis, Antarctica/McMurdo, Asia/Macao, America/Manaus, Africa/Freetown, Europe/Bucharest, Asia/Tomsk, America/Argentina/Mendoza, Asia/Macau, Europe/Malta, Mexico/BajaSur, Pacific/Tahiti, Africa/Asmera, Europe/Busingen, America/Argentina/Rio_Gallegos, Africa/Malabo, Europe/Skopje, America/Catamarca, America/Godthab, Europe/Sarajevo, Australia/ACT, GB-Eire, Africa/Lagos, America/Cordoba, Europe/Rome, Asia/Dacca, Indian/Mauritius, Pacific/Samoa, America/Regina, America/Fort_Wayne, America/Dawson_Creek, Africa/Algiers, Europe/Mariehamn, America/St_Johns, America/St_Thomas, Europe/Zurich, America/Anguilla, Asia/Dili, America/Denver, Africa/Bamako, Europe/Saratov, GB, Mexico/General, Pacific/Wallis, Europe/Gibraltar, Africa/Conakry, Africa/Lubumbashi, Asia/Istanbul, America/Havana, NZ-CHAT, Asia/Choibalsan, America/Porto_Acre, Asia/Omsk, Europe/Vaduz, US/Michigan, Asia/Dhaka, America/Barbados, Europe/Tiraspol, Atlantic/Cape_Verde, Asia/Yekaterinburg, America/Louisville, Pacific/Johnston, Pacific/Chatham, Europe/Ljubljana, America/Sao_Paulo, Asia/Jayapura, America/Curacao, Asia/Dushanbe, America/Guyana, America/Guayaquil, America/Martinique, Portugal, Europe/Berlin, Europe/Moscow, Europe/Chisinau, America/Puerto_Rico, America/Rankin_Inlet, Pacific/Ponape, Europe/Stockholm, Europe/Budapest, America/Argentina/Jujuy, Australia/Eucla, Asia/Shanghai, Universal, Europe/Zagreb, America/Port_of_Spain, Europe/Helsinki, Asia/Beirut, Asia/Tel_Aviv, Pacific/Bougainville, US/Central, Africa/Sao_Tome, Indian/Chagos, America/Cayenne, Asia/Yakutsk, Pacific/Galapagos, Australia/North, Europe/Paris, Africa/Ndjamena, Pacific/Fiji, America/Rainy_River, Indian/Maldives, Australia/Yancowinna, SystemV/AST4, Asia/Oral, America/Yellowknife, Pacific/Enderbury, America/Juneau, Australia/Victoria, America/Indiana/Vevay, Asia/Tashkent, Asia/Jakarta, Africa/Ceuta, Asia/Barnaul, America/Recife, America/Buenos_Aires, America/Noronha, America/Swift_Current, Australia/Adelaide, America/Metlakatla, Africa/Djibouti, America/Paramaribo, Asia/Qostanay, Europe/Simferopol, Europe/Sofia, Africa/Nouakchott, Europe/Prague, America/Indiana/Vincennes, Antarctica/Mawson, America/Kralendijk, Antarctica/Troll, Europe/Samara, Indian/Christmas, America/Antigua, Pacific/Gambier, America/Indianapolis, America/Inuvik, America/Iqaluit, Pacific/Funafuti, UTC, Antarctica/Macquarie, Canada/Pacific, America/Moncton, Africa/Gaborone, Pacific/Chuuk, Asia/Pyongyang, America/St_Vincent, Asia/Gaza, Etc/Universal, PST8PDT, Atlantic/Faeroe, Asia/Qyzylorda, Canada/Newfoundland, America/Kentucky/Louisville, America/Yakutat, Asia/Ho_Chi_Minh, Antarctica/Casey, Europe/Copenhagen, Africa/Asmara, Atlantic/Azores, Europe/Vienna, ROK, Pacific/Pitcairn, America/Mazatlan, Australia/Queensland, Pacific/Nauru, Europe/Tirane, Asia/Kolkata, SystemV/MST7, Australia/Canberra, MET, Australia/Broken_Hill, Europe/Riga, America/Dominica, Africa/Abidjan, America/Mendoza, America/Santarem, Kwajalein, America/Asuncion, Asia/Ulan_Bator, NZ, America/Boise, Australia/Currie, EST5EDT, Pacific/Guam, Pacific/Wake, Atlantic/Bermuda, America/Costa_Rica, America/Dawson, Asia/Chongqing, Eire, Europe/Amsterdam, America/Indiana/Knox, America/North_Dakota/Beulah, Africa/Accra, Atlantic/Faroe, Mexico/BajaNorte, America/Maceio, Etc/UCT, Pacific/Apia, GMT0, America/Atka, Pacific/Niue, Australia/Lord_Howe, Europe/Dublin, Pacific/Truk, MST7MDT, America/Monterrey, America/Nassau, America/Jamaica, Asia/Bishkek, America/Atikokan, Atlantic/Stanley, Australia/NSW, US/Hawaii, SystemV/CST6, Indian/Mahe, Asia/Aqtobe, America/Sitka, Asia/Vladivostok, Africa/Libreville, Africa/Maputo, Zulu, America/Kentucky/Monticello, Africa/El_Aaiun, Africa/Ouagadougou, America/Coral_Harbour, Pacific/Marquesas, Brazil/West, America/Aruba, America/North_Dakota/Center, America/Cayman, Asia/Ulaanbaatar, Asia/Baghdad, Europe/San_Marino, America/Indiana/Tell_City, America/Tijuana, Pacific/Saipan, SystemV/YST9, Africa/Douala, America/Chihuahua, America/Ojinaga, Asia/Hovd, America/Anchorage, Chile/EasterIsland, America/Halifax, Antarctica/Rothera, America/Indiana/Indianapolis, US/Mountain, Asia/Damascus, America/Argentina/San_Luis, America/Santiago, Asia/Baku, America/Argentina/Ushuaia, Atlantic/Reykjavik, Africa/Brazzaville, Africa/Porto-Novo, America/La_Paz, Antarctica/DumontDUrville, Asia/Taipei, Antarctica/South_Pole, Asia/Manila, Asia/Bangkok, Africa/Dar_es_Salaam, Poland, Atlantic/Madeira, Antarctica/Palmer, America/Thunder_Bay, Africa/Addis_Ababa, Asia/Yangon, Europe/Uzhgorod, Brazil/DeNoronha, Asia/Ashkhabad, Etc/Zulu, America/Indiana/Marengo, America/Creston, America/Punta_Arenas, America/Mexico_City, Antarctica/Vostok, Asia/Jerusalem, Europe/Andorra, US/Samoa, PRC, Asia/Vientiane, Pacific/Kiritimati, America/Matamoros, America/Blanc-Sablon, Asia/Riyadh, Iceland, Pacific/Pohnpei, Asia/Ujung_Pandang, Atlantic/South_Georgia, Europe/Lisbon, Asia/Harbin, Europe/Oslo, Asia/Novokuznetsk, CST6CDT, Atlantic/Canary, America/Knox_IN, Asia/Kuwait, SystemV/HST10, Pacific/Efate, Africa/Lome, America/Bogota, America/Menominee, America/Adak, Pacific/Norfolk, Europe/Kirov, America/Resolute, Pacific/Tarawa, Africa/Kampala, Asia/Krasnoyarsk, Greenwich, SystemV/EST5, America/Edmonton, Europe/Podgorica, Australia/South, Canada/Central, Africa/Bujumbura, America/Santo_Domingo, US/Eastern, Europe/Minsk, Pacific/Auckland, Africa/Casablanca, America/Glace_Bay, Canada/Eastern, Asia/Qatar, Europe/Kiev, Singapore, Asia/Magadan, SystemV/PST8, America/Port-au-Prince, Europe/Belfast, America/St_Barthelemy, Asia/Ashgabat, Africa/Luanda, America/Nipigon, Atlantic/Jan_Mayen, Brazil/Acre, Asia/Muscat, Asia/Bahrain, Europe/Vilnius, America/Fortaleza, Etc/GMT0, US/East-Indiana, America/Hermosillo, America/Cancun, Africa/Maseru, Pacific/Kosrae, Africa/Kinshasa, Asia/Kathmandu, Asia/Seoul, Australia/Sydney, America/Lima, Australia/LHI, America/St_Lucia, Europe/Madrid, America/Bahia_Banderas, America/Montserrat, Asia/Brunei, America/Santa_Isabel, Canada/Mountain, America/Cambridge_Bay, Asia/Colombo, Australia/West, Indian/Antananarivo, Australia/Brisbane, Indian/Mayotte, US/Indiana-Starke, Asia/Urumqi, US/Aleutian, Europe/Volgograd, America/Lower_Princes, America/Vancouver, Africa/Blantyre, America/Rio_Branco, America/Danmarkshavn, America/Detroit, America/Thule, Africa/Lusaka, Asia/Hong_Kong, Iran, America/Argentina/La_Rioja, Africa/Dakar, SystemV/CST6CDT, America/Tortola, America/Porto_Velho, Asia/Sakhalin, Etc/GMT+10, America/Scoresbysund, Asia/Kamchatka, Asia/Thimbu, Africa/Harare, Etc/GMT+12, Etc/GMT+11, Navajo, America/Nome, Europe/Tallinn, Turkey, Africa/Khartoum, Africa/Johannesburg, Africa/Bangui, Europe/Belgrade, Jamaica, Africa/Bissau, Asia/Tehran, WET, Europe/Astrakhan, Africa/Juba, America/Campo_Grande, America/Belem, Etc/Greenwich, Asia/Saigon, America/Ensenada, Pacific/Midway, America/Jujuy, Africa/Timbuktu, America/Bahia, America/Goose_Bay, America/Virgin, America/Pangnirtung, Asia/Katmandu, America/Phoenix, Africa/Niamey, America/Whitehorse, Pacific/Noumea, Asia/Tbilisi, America/Montreal, Asia/Makassar, America/Argentina/San_Juan, Hongkong, UCT, Asia/Nicosia, America/Indiana/Winamac, SystemV/MST7MDT, America/Argentina/ComodRivadavia, America/Boa_Vista, America/Grenada, Asia/Atyrau, Australia/Darwin, Asia/Khandyga, Asia/Kuala_Lumpur, Asia/Famagusta, Asia/Thimphu, Asia/Rangoon, Europe/Bratislava, Asia/Calcutta, America/Argentina/Tucuman, Asia/Kabul, Indian/Cocos, Japan, Pacific/Tongatapu, America/New_York, Etc/GMT-12, Etc/GMT-11, America/Nuuk, Etc/GMT-10, SystemV/YST9YDT, Europe/Ulyanovsk, Etc/GMT-14, Etc/GMT-13, W-SU, America/Merida, EET, America/Rosario, Canada/Saskatchewan, America/St_Kitts, Arctic/Longyearbyen, America/Fort_Nelson, America/Caracas, America/Guadeloupe, Asia/Hebron, Indian/Kerguelen, SystemV/PST8PDT, Africa/Monrovia, Asia/Ust-Nera, Egypt, Asia/Srednekolymsk, America/North_Dakota/New_Salem, Asia/Anadyr, Australia/Melbourne, Asia/Irkutsk, America/Shiprock, America/Winnipeg, Europe/Vatican, Asia/Amman, Etc/UTC, SystemV/AST4ADT, Asia/Tokyo, America/Toronto, Asia/Singapore, Australia/Lindeman, America/Los_Angeles, SystemV/EST5EDT, Pacific/Majuro, America/Argentina/Buenos_Aires, Europe/Nicosia, Pacific/Guadalcanal, Europe/Athens, US/Pacific, Europe/Monaco]
ZoneId.systemDefault()： Asia/Shanghai
ZoneId.of("Asia/Shanghai"): Asia/Shanghai
========================时区ZonedDateTime, OffsetDateTime===================================
ZonedDateTime.of() Shanghai, 2022-01-02T19:22:20.313885+08:00[Asia/Shanghai]
ZonedDateTime.of() America, 2022-01-02T19:22:20.314281-04:00[America/Cuiaba]
OffsetDateTime.of(), -42022-01-02T19:22:20.315687-04:00
OffsetDateTime.of(), UTC2022-01-02T19:22:20.316207Z
=======================Clock==================================================
System.currentTimeMillis(): 1641122540316
Clock.systemDefaultZone(): SystemClock[Asia/Shanghai]
Clock.systemUTC(): SystemClock[Z]
Clock systemDefaultZone millis: 1641122540317
Clock systemUTC millis: 1641122540317
Clock systemDefaultZone instant: 2022-01-02T11:22:20.317917Z
Clock systemUTC instant: 2022-01-02T11:22:20.318222Z
LocalDateTime.now with Clock.systemDefaultZone()2022-01-02T19:22:20.318516
LocalDateTime.now with Clock.systemUTC()2022-01-02T11:22:20.318801
LocalDateTime.now with zoneId default: 2022-01-02T19:22:20.319114
LocalDateTime.now with zoneId of: 2022-01-02T07:22:20.319415

LocalDate等新增类

Instant代替 Date，LocalDateTime代替 Calendar，DateTimeFormatter 代替 SimpleDateFormat。

• LocalDate
• LocalTime
• LocalDateTime

它们定义了一个没有时区信息的时间，默认是本地运行所在时区的时间，通过使用atZone、Clock参数等方式可得到相应时区的时间。

• Instant

Instant用于表示一个时间戳，相对于UTC时间
toInstant()方法会进行转换：The calculation combines the local date-time and offset.

• Duration
• Period：Period在概念上和Duration类似，区别在于Period是以年月日来衡量一个时间段，比如2年3个月6天
• Clock

Clock 类提供了访问当前日期和时间的方法，Clock 是时区敏感的。如果你需要对不同时区的日期进行处理的话这是相当方便的，LocalDateTime.now(Clock.system(ZoneId.of("xxx")))。

增加和减少日期

• with()、plus等

• TemporalAdjuster接口

  // 如果TemporalAdjusters类的方法不能满足使用，可以创建自定义的TemporalAdjuster接口的实现
  @FunctionalInterface
  public interface TemporalAdjuster {
    Temporal adjustInto(Temporal temporal);
  }

• TemporalAdjusters类： | 方法名 | 描述 | | —- | —- | | firstInMonth | 返回同一个月中第一个星期几 | | nextOrSame / previousOrSame | 返回后一个/前一个给定的星期几，如果这个值满足条件，直接返回 |

Duration和Period

• Duration表示一个时间段，所以Duration类中不包含now()静态方法。可以通过Duration.between()方法创建Duration对象。
• Period在概念上和Duration类似，区别在于Period是以年月日来衡量一个时间段。

  格式化日期DateTimeFormatter

  format和parse

时区

ZoneId
ZonedDateTime、OffsetDateTime

Clock应用于单元测试

    /**
     * Clock.fixed
     * Obtains a clock that always returns the same instant.
     * This clock simply returns the specified instant. As such, it is not a clock in the conventional sense. The main use case for this is in testing, where the fixed clock ensures tests are not dependent on the current clock.
     * The returned implementation is immutable, thread-safe and Serializable.
     * Params:
     * fixedInstant – the instant to use as the clock, not null
     * zone – the time-zone to use to convert the instant to date-time, not null
     * Returns:
     * a clock that always returns the same instant, not null
     * @param localTime
     * @return
     */
private Clock createFixedClock(LocalTime localTime) {
    ZoneId zoneId = ZoneId.systemDefault();
    Instant instant = localTime.atDate(LocalDate.now()).atZone(zoneId).toInstant();
    return Clock.fixed(instant, zoneId);
}

与Date的相互转换

转换demo
Date -> LocalDateTime

@Test
public void test() {
    // 关键步骤：
    Instant instant = date.toInstant();
    LocalDateTime localDateTime = LocalDateTime.ofInstant(instant, zone);
}

LocalDateTime -> Date

@Test
public void test() {
    // 关键步骤：
    Instant instant = localDateTime.atZone(zone).toInstant();
    java.util.Date date = Date.from(instant);
}

案例

// 距离过期还有不到一个月
public boolean hasReminderOfExpireDate() {
    LocalDate now = LocalDate.now();
    return remain > 0 && expiredDate.isBefore(now.plusDays(31));
}

Date

SimpleDateFormat

new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.US)
simpleDateFormat.setTimeZone(TimeZone.getDefault());
simpleDateFormat.format(date);
simpleDateFormat.parse();

String dateString = "2020-02-12 20:55:09";
DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date date = null;
try {
    date = df.parse(dateString);
} catch (ParseException e) {
    e.printStackTrace();
}

Date currentTime = new Date();
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString = formatter.format(currentTime);

Calendar

new GregorianCalendar(2021, Calendar.JUNE, 8).getTime()

TimeZone

TimeZone.getAvailableIDs();
TimeZone.getTimeZone("GMT+08:00").getID();
TimeZone.getTimeZone("America/New_York").getID();
TimeZone.setDefault(TimeZone.getTimeZone("GMT+06:00"));

https://24timezones.com/time-zones