Date

The Date is another built-in object in JavaScript. We used it in the development of the SleepTime App. Here is a summary of its operations. For a more detailed reference, visit MDN web docs.

Constructing Dates

• Date measured in "milliseconds" (adjusted for leap seconds) since the "epoch" (midnight of January 1, 1970 UTC).

 
 
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• You can pass milliseconds to the constructor; valid range ±100,000,000 days from epoch.

 
 
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Caution: Dates are converted to numbers in arithmetic expressions.

• You can pass the following parameters to Date() constructor to create a specific date:

  • year,
  • zeroBasedMonth,
  • day,
  • hours,
  • minutes,
  • seconds,
  • milliseconds

• The parameters must be provided in order, and everything starting from "day" are optional.

 
 
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Caution: Out-of-range zeroBasedMonth, day, hours, etc. silently roll.

 
 
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• For UTC, construct with date string in format YYYY-MM-DDTHH:mm:ss.sssZ (with literal T, Z):

 
 
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Caution: Other string formats may be accepted by the constructor, but their format is not standardized.

Caution: Date(...) without new constructs string (in non-standard format).

 
 
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Static Date Functions

• Date.now() is the current date/time, but it yields milliseconds, not a Date object.

 
 
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• A very useful static function is Date.parse(dateString). It also yields milliseconds, but you can pass that to the Date constructor to get a Date object.

 
 
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Caution: support for dateString in YYYY-MM-DDTHH:mm:ss.sssZ format is guaranteed. Support for other formats are implementation-dependent.

• Another useful static function is Date.UTC(year, zeroBasedMonth, day, hours, minutes, seconds, milliseconds) where arguments after year are optional.

Getters and Setters

• The Date object has traditional getter/setter methods getHours/setHours.

 
 
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• For UTC, there are UTC variants getUTCFullYear, setUTCFullYear, getUTCMonth, setUTCMonth, and so on.

Date Formatting

• toISOString yields string in YYYY-MM-DDTHH:mm:ss.sssZ format.

 
 
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• toString, toDateString, toTimeString yield "humanly readable" string in local time zone, or only the date/time portion:

 
 
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• toUTCString yields "humanly readable" string in UTC:

 
 
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• toLocaleString, toLocaleDateString, toLocaleTimeString yield localized string:

 
 
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