javascript的date操做使用

Date

Date 對象用於處理日期與時間。

構造方法

new Date();
new Date(value);
new Date(dateString);
new Date(year, month[, day[, hour[, minutes[, seconds[, milliseconds]]]]]);

參數：

value
表明自1970年1月1日00:00:00 (世界標準時間) 起通過的毫秒數。
dateString
表示日期的字符串值。該字符串應該能被 Date.parse() 方法識別（符合 IETF-compliant RFC 2822 timestamps 或 version of ISO8601）。
year
表明年份的整數值。爲了不2000年問題最好指定4位數的年份; 使用 1998, 而不要用 98.
month
表明月份的整數值從0（1月）到11（12月）。
day
表明一個月中的第幾天的整數值，從1開始。
hour
表明一天中的小時數的整數值 (24小時制)。
minute
分鐘數。
second
秒數。
millisecond
表示時間的毫秒部分的整數值。

須要注意的是隻能經過調用 Date 構造函數來實例化日期對象：以常規函數調用它（即不加 new 操做符）將會返回一個字符串，而不是一個日期對象。另外，不像其餘JavaScript 類型，Date 對象沒有字面量格式。
當Date做爲構造函數調用並傳入多個參數時，若是數值大於合理範圍時（如月份爲13或者分鐘數爲70），相鄰的數值會被調整。好比 new Date(2013, 13, 1)等於new Date(2014, 1, 1)，它們都表示日期2014-02-01（注意月份是從0開始的）。其餘數值也是相似，new Date(2013, 2, 1, 0, 70)等於new Date(2013, 2, 1, 1, 10)，都表示時間2013-03-01T01:10:00。

• 若是沒有輸入任何參數，則Date的構造器會依據系統設置的當前時間來建立一個Date對象。
• 若是提供了至少兩個參數，其他的參數均會默認設置爲1（若是沒有提供day參數）或者0。
• JavaScript的時間是由世界標準時間（UTC）1970年1月1日開始，用毫秒計時，一天由86,400,000毫秒組成。Date對象的範圍是-100,000,000天至100,000,000天（等效的毫秒值）。
• JavaScript的Date對象爲跨平臺提供了統一的行爲。時間屬性能夠在不一樣的系統中表示相同的時刻，而若是使用了本地時間對象，則反映當地的時間。
• JavaScript 的Date對象提供了數個UTC時間的方法，也相應提供了當地時間的方法。UTC，也就是咱們所說的格林威治時間，指的是time中的世界時間標準。而當地時間則是指執行JavaScript的客戶端電腦所設置的時間。
• 以一個函數的形式來調用JavaScript的Date對象（i.e., 不使用 new 操做符）會返回一個表明當前日期和時間的字符串。

dateString

表示日期的字符串值。該字符串應該能被 Date.parse() 方法識別（符合 IETF-compliant RFC 2822 timestamps 或 version of ISO8601）。因此須要確認 datestring的類型。

IETF-compliant RFC 2822 timestamps

最初的datestring的只支持：時間字符串

Date and Time Specification

   Date and time occur in several header fields.  This section specifies
   the syntax for a full date and time specification.  Though folding
   white space is permitted throughout the date-time specification, it
   is RECOMMENDED that a single space be used in each place that FWS
   appears (whether it is required or optional); some older
   implementations may not interpret other occurrences of folding white
   space correctly.

date-time       =       [ day-of-week "," ] date FWS time [CFWS]

day-of-week     =       ([FWS] day-name) / obs-day-of-week

day-name        =       "Mon" / "Tue" / "Wed" / "Thu" /
                        "Fri" / "Sat" / "Sun"

date            =       day month year

year            =       4*DIGIT / obs-year

month           =       (FWS month-name FWS) / obs-month

month-name      =       "Jan" / "Feb" / "Mar" / "Apr" /
                        "May" / "Jun" / "Jul" / "Aug" /
                        "Sep" / "Oct" / "Nov" / "Dec"

day             =       ([FWS] 1*2DIGIT) / obs-day

time            =       time-of-day FWS zone

time-of-day     =       hour ":" minute [ ":" second ]

hour            =       2DIGIT / obs-hour

minute          =       2DIGIT / obs-minute

second          =       2DIGIT / obs-second

zone            =       (( "+" / "-" ) 4DIGIT) / obs-zone

The day is the numeric day of the month.  The year is any numeric
   year 1900 or later.

   The time-of-day specifies the number of hours, minutes, and
   optionally seconds since midnight of the date indicated.

   The date and time-of-day SHOULD express local time.

   The zone specifies the offset from Coordinated Universal Time (UTC,
   formerly referred to as "Greenwich Mean Time") that the date and
   time-of-day represent.  The "+" or "-" indicates whether the
   time-of-day is ahead of (i.e., east of) or behind (i.e., west of)
   Universal Time.  The first two digits indicate the number of hours
   difference from Universal Time, and the last two digits indicate the
   number of minutes difference from Universal Time.  (Hence, +hhmm
   means +(hh * 60 + mm) minutes, and -hhmm means -(hh * 60 + mm)
   minutes).  The form "+0000" SHOULD be used to indicate a time zone at
   Universal Time.  Though "-0000" also indicates Universal Time, it is
   used to indicate that the time was generated on a system that may be
   in a local time zone other than Universal Time and therefore
   indicates that the date-time contains no information about the local
   time zone.

   A date-time specification MUST be semantically valid.  That is, the
   day-of-the-week (if included) MUST be the day implied by the date,
   the numeric day-of-month MUST be between 1 and the number of days
   allowed for the specified month (in the specified year), the
   time-of-day MUST be in the range 00:00:00 through 23:59:60 (the
   number of seconds allowing for a leap second; see [STD12]), and the
   zone MUST be within the range -9959 through +9959.

因此能夠得出結論：最開始的js僅支持如下格式的datestring【須要注意的是各個瀏覽器實現不一致，因此可能會支持其餘的格式的datestring】

Sun 30 Dec 2018 19:42:44 +0800

看起來和console.log裏面的輸出有一點不一致；

Sun Dec 30 2018 19:42:44 GMT+0800 (中國標準時間)

version of ISO8601

在es5開始，添加了ISO8601格式的支持；

Date Time String Format
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYY    is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.

-    「-」 (hyphen) appears literally twice in the string.

MM    is the month of the year from 01 (January) to 12 (December).

DD    is the day of the month from 01 to 31.

T    「T」 appears literally in the string, to indicate the beginning of the time element.

HH    is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.

:    「:」 (colon) appears literally twice in the string.

mm    is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.

ss    is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.

.    「.」 (dot) appears literally in the string.

sss    is the number of complete milliseconds since the start of the second as three decimal digits.

Z    is the time zone offset specified as 「Z」 (for UTC) or either 「+」 or 「-」 followed by a time expression HH:mm

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD
It also includes 「date-time」 forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base 10. If the MM or DD fields are absent 「01」 is used as the value. If the HH, mm, or ss fields are absent 「00」 is used as the value and the value of an absent sss field is 「000」. The value of an absent time zone offset is 「Z」.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.

NOTE 1 As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00

NOTE 2 There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.

datestring的格式能夠是以下：

• [date]
• [date]T[time]
• [date]T[time]Z
• [date]Ttime[0800]

其中[date]能夠是：

• YYYY
• YYYY-MM
• YYYY-MM-DD

而[time]能夠是一下：

• THH:mm
• THH:mm:ss
• THH:mm:ss.sss

能夠說[date]是必須有的，其餘的都是能夠沒有，會取默認值，固然各個瀏覽器內部實現的不一致，因此可能還會出現不一致的問題；

API列表

Date.UTC()
Date.now()
Date.parse()
Date.prototype.getDate()
Date.prototype.getDay()
Date.prototype.getFullYear()
Date.prototype.getHours()
Date.prototype.getMilliseconds()
Date.prototype.getMinutes()
Date.prototype.getMonth()
Date.prototype.getSeconds()
Date.prototype.getTime()
Date.prototype.getTimezoneOffset()
Date.prototype.getUTCDate()
Date.prototype.getUTCDay()
Date.prototype.getUTCFullYear()
Date.prototype.getUTCHours()
Date.prototype.getUTCMilliseconds()
Date.prototype.getUTCMinutes()
Date.prototype.getUTCMonth()
Date.prototype.getUTCSeconds()
Date.prototype.getYear()
Date.prototype.setDate()
Date.prototype.setFullYear()
Date.prototype.setHours()
Date.prototype.setMilliseconds()
Date.prototype.setMinutes()
Date.prototype.setMonth()
Date.prototype.setSeconds()
Date.prototype.setTime()
Date.prototype.setUTCDate()
Date.prototype.setUTCFullYear()
Date.prototype.setUTCHours()
Date.prototype.setUTCMilliseconds()
Date.prototype.setUTCMinutes()
Date.prototype.setUTCMonth()
Date.prototype.setUTCSeconds()
Date.prototype.setYear()
Date.prototype.toDateString()
Date.prototype.toGMTString()
Date.prototype.toISOString()
Date.prototype.toJSON()
Date.prototype.toLocaleDateString()
Date.prototype.toLocaleFormat()
Date.prototype.toLocaleString()
Date.prototype.toLocaleTimeString()
Date.prototype.toSource()
Date.prototype.toString()
Date.prototype.toTimeString()
Date.prototype.toUTCString()
Date.prototype.valueOf()
Date.prototype[@@toPrimitive]

一元加法|加法

+new Date()
// 1546335448125
0+new Date()
// "0Tue Jan 01 2019 17:53:17 GMT+0800 (中國標準時間)"

二元加法獲取date的值；

加法運算

上圖中的NOTE 1說，在步驟5和6中的ToPrimitive調用沒有提供hint，除了Date對象以外全部ECMAScript對象將缺乏hint的狀況當作Number處理；

js各個類型的toPrimitive

而本文須要看的是date[@@toPrimitive]() 方法能夠轉換一個 Date 對象到一個原始值。

Date()[Symbol.toPrimitive](hint)

==>date.toPrimitive;

給出的 Date 的原始值。根據傳入參數的不一樣，能夠返回 string 或 number 類型。

Date 對象的 [@@toPrimitive]() 方法能夠返回一個原始值，或是 string，或是number。

若是 hint 是 "string" 或 "default"，[@@toPrimitive]() 將會調用 toString。若是 toString 屬性不存在，則調用 valueOf。若是 valueOf 也不存在，則拋出一個TypeError。

若是 hint 是 "number"，[@@toPrimitive]() 會首先嚐試 valueOf，若失敗再嘗試 toString。

當指望一個原始值卻收到一個對象時，JavaScript 能夠自動的調用 [@@toPrimitive]() 方法來將一個對象轉化成原始值，因此你不多會須要本身調用這個方法。

date的toPrimitive調用，不傳遞hint的時候，默認是string；

因此：

0+new Date()
// "0Tue Jan 01 2019 17:53:17 GMT+0800 (中國標準時間)"

一元加法獲取date的值

'+'號運算符做爲一元運算符時，Expression將進行ToNumber操做。

能夠看到，此時date的toPrimitive調用，傳遞hint是number，返回毫秒數；

參考： JavaScript 加號運算符詳解