Convert a date to a string using a pattern as a template.

Date.toFormattedString
  "EEEE, MMMM d, y 'at' h:mm a"
  (Date.fromParts 2007 Mar 15 13 45 56 67)
-- "Thursday, March 15, 2007 at 1:45 PM"

Each alphabetic character in the pattern represents date or time information; the number of times a character is repeated specifies the form of the name to use (e.g. "Tue", "Tuesday") or the padding of numbers (e.g. "1", "01"). Formatting characters are escaped within single-quotes, and a single-quote is escaped as a sequence of two single-quotes, whether appearing inside or outside an escaped sequence.

Patterns are based on Date Format Patterns in [Unicode Technical Standard #35](http://www.unicode.org/reports/tr35/tr35-43/ tr35-dates.html#Date_Format_Patterns). Only the following subset of formatting characters are available:

"y" -- year
"Y" -- week-numbering year
"Q" -- quarter
"M" -- month
"w" -- week number
"d" -- day
"D" -- ordinal day
"E" -- day of week
"e" -- weekday number / day of week
"a" -- day period (AM, PM)
"b" -- day period (am, pm, noon, midnight)
"h" -- hour (12-hour clock)
"H" -- hour (24-hour clock)
"m" -- minute
"s" -- second
"S" -- fractional second
"X" -- time offset, using "Z" when offset is 0
"x" -- time offset

The non-standard pattern field "ddd" is available to indicate the day of the month with an ordinal suffix (e.g. "1st", "15th"), as the current standard does not include such a field.

Date.toFormattedString
  "MMMM ddd, y"
  (Date.fromParts 2007 Mar 15 13 45 56 67)
-- "March 15th, 2007"

Convenience function for formatting a date to ISO 8601 (extended date and time format with local time offset).

Date.toIsoString (Date.fromParts 2007 Mar 15 13 45 56 67)
-- "2007-03-15T13:45:56.067-04:00"
-- (example has a local offset of UTC-04:00)

Convert a date to a string just like toFormattedString, but using the UTC representation instead of the local representation of the date.

Convenience function for formatting a date, in UTC representation, to ISO 8601 (extended date and time format with "Z" for time offset).

Date.toUtcIsoString (Date.fromParts 2007 Mar 15 13 45 56 67)
-- "2007-03-15T17:45:56.067Z"
-- (example has a local offset of UTC-04:00)

Convert a date to a Julian Date. Julian Dates always represent UTC time.

Date.toJulianDate (Date.fromSpec utc noTime (calendarDate 2000 Jan 1))
-- 2451544.5