UTS- Date format patterns

http://unicode.org/reports/tr35/tr35-6.html#Date_Format_Patterns

Appendix F: Date Format
Patterns

A date pattern is a string of characters, where specific strings of
characters are replaced with date and time data from a calendar when formatting
or used to generate data for a calendar when parsing. The following are the
characters used in patterns to show the appropriate formats for a given locale.
The following are examples:

Characters may be used multiple times. For example, if y is used for the
year, ‘yy‘ might produce ‘99‘, whereas ‘yyyy‘ produces ‘1999‘. For most
numerical fields, the number of characters specifies the field width. For
example, if h is the hour, ‘h‘ might produce ‘5‘, but ‘hh‘ produces ‘05‘. For
some characters, the count specifies whether an abbreviated or full form should
be used, but may have other choices, as given below.

Two single quotes represents a literal single quote, either inside or outside
single quotes. Text within single quotes is not interpreted in any way (except
for two adjacent single quotes). Otherwise all ASCII letter from a to z and A to
Z are reserved as syntax characters, and require quoting if they are to
represent literal characters. In addition, certain ASCII punctuation characters
may become variable in the future (eg ":" being interpreted as the time
separator and ‘/‘ as a date separator, and replaced by respective
locale-sensitive characters in display).

Note: the counter-intuitive use of 5 letters for the narrow form of
weekdays and months is forced by backwards compatibility.

All non-letter character represent themselves in a pattern, except for the
single quote. It is used to ‘escape‘ letters. Two single quotes in a row,
whether inside or outside a quoted sequence, represent a ‘real‘ single
quote.

F.1 Localized Pattern
Characters (deprecated)

These are characters that can be used when displaying a date pattern to an
end user. This can occur, for example, when a spreadsheet allows users to
specify date patterns. Whatever is in the string is substituted one-for-one with
the characters "GyMdkHmsSEDFwWahKzYe", with the above
meanings. Thus, for example, if "J" is to be used instead of "Y" to mean
Year, then the string would be: "GyMdkHmsSEDFwWahKzJe".

This element is deprecated. It is recommended instead that a more
sophisticated UI be used for localization, such as using icons to represent the
different formats (and lengths) in the Date
Field Symbol Table.

F.2 AM / PM

Even for countries where the customary date format only has a 24 hour format,
both the am and pm localized strings must be present and must be distinct from
one another. Note that as long as the 24 hour format is used, these strings will
normally never be used, but for testing and unusual circumstances they must be
present.

F.3 Eras

There are only two values for an era in a Gregorian calendar, "BC" and "AD".
These values can be translated into other languages, like "a.C." and and "d.C."
for Spanish, but there are no other eras in the Gregorian calendar. Other
calendars have a different numbers of eras. Care should be taken when
translating the era names for a specific calendar.

F.4 Week of Year

Values calculated for the Week of Year field range from 1 to 53 for the
Gregorian calendar (they may have different ranges for other calendars). Week 1
for a year is the first week that contains at least the specified minimum number
of days from that year. Weeks between week 1 of one year and week 1 of the
following year are numbered sequentially from 2 to 52 or 53 (if needed). For
example, January 1, 1998 was a Thursday. If the first day of the week is MONDAY
and the minimum days in a week is 4 (these are the values reflecting ISO 8601
and many national standards), then week 1 of 1998 starts on December 29, 1997,
and ends on January 4, 1998. However, if the first day of the week is SUNDAY,
then week 1 of 1998 starts on January 4, 1998, and ends on January 10, 1998. The
first three days of 1998 are then part of week 53 of 1997.

Values are similarly calculated for the Week of Month.

F.5 Week Elements

firstDay

A number indicating which day of the week is considered the ‘first‘ day,
for calendar purposes. Because the ordering of days may vary between calendar,
keywords are used for this value, such as sun, mon,... These values will be
replaced by the localized name when they are actually used.

minDays (Minimal Days in First Week)

Minimal days required in the first week of a month or year. For example,
if the first week is defined as one that contains at least one day, this value
will be 1. If it must contain a full seven days before it counts as the first
week, then the value would be 7.

weekendStart, weekendEnd

Indicates the day and time that the weekend starts or ends. As with
firstDay, keywords are used instead of numbers.