Date

Date objects are represented internally by a timestamp, which is milliseconds passed since the epoch (ie. midnight 01 January, 1970 UTC)

There are always two ways to interpret a timestamp:

1. as a local time
   • The local timezone is not stored in the date object, but is determined by the host environment.
2. as a Coordinated Universal Time (UTC)

String format

The JavaScript specification only specifies one string format to be universally supported: YYYY-MM-DDTHH:mm:ss.sssZ

• T is a literal character, which indicates the beginning of the time part of the string
• Z is the timezone offset, which can either be the literal character Z (indicating UTC), or + or - followed by HH:mm, the offset in hours and minutes from UTC.
  • When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as local time (see below for date-only and date-time form explanation)

Various components can be omitted, so the following are all valid:

• Date-only form: YYYY, YYYY-MM, YYYY-MM-DD
• Date-time form: one of the above date-only forms, followed by T, followed by HH:mm, HH:mm:ss, or HH:mm:ss.sss. Each combination can be followed by a time zone offset.

Converting a Date object to string

• Date.toISOString() will return a string in the format: 1970-01-01T00:00:00.000Z
  • note: this is the same string format that the Date object accepts as input
• Date.toString() will return a string in the format Thu Jan 01 1970 00:00:00 GMT+0000
• Date.toDateString() and Date.toTimeString() return the date and time parts of the string, respectively.
• Date.toLocaleDateString(), Date.toLocaleTimeString(), and Date.toLocaleString() use locale-specific date and time formats, usually provided by the Intl API.

---

months are zero-based, so December is 11

Date.now() vs new Date()

new Date() - creates a Date object representing the current (or passed) date/time Date.now() - returns the number of milliseconds since the epoch

• we also get this with new Date().valueOf() and new Date().getTime()

As a general rule of thumb, use...

• Date.now() in code that deals with intervals (usually subtracting an earlier date from a later date to calculate the time elapsed)
• new Date() for timestamps, e.g. those written to a database.

---

Children

1. Cook