XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (562 page)

Character	Resulting Format	Example
1	Decimal numeric format with no leading zeroes: 1, 2, 3, …	1, 2, 3
01	Decimal format, two digits: 01, 02, 03, …	01, 02, 03
Other Unicode digit	Decimal numeric format, using the set of Unicode digits in which the given digit represents the value one.
i	Lower-case Roman numerals.	i, ii, iii, iv
I	Upper-case Roman numerals.	I, II, III, IV
N	Name of component, in upper case.	MONDAY, TUESDAY
n	Name of component, in lower case.	monday, tuesday
Nn	Name of component, in title case.	Monday, Tuesday
W	Number expressed in upper case words.	ONE, TWO, THREE
w	Number expressed in lower case words.	one, two, three
Ww	Number expressed in title case words.	One, Two, Three
t	Indicates so-called traditional numbering. The meaning of this depends on the language; it is intended to produce the same effect as letter-value=“traditional” in . The requirement most often cited is for Hebrew numbering.
o	Indicates ordinal numbering. For example, 1o gives 1st , 2nd , 3rd … , while wo gives first , second , third , and Wwo gives First , Second , Third . All these examples are of course language dependent: the language is controlled using the language argument.

Not all combinations of these make sense, but the specification leaves it very open to implementations how to interpret combinations other than the obvious ones. The golden rule for this function is that so long as the syntax of the picture is correct, it is never supposed to raise an error—if the processor doesn't understand the format that you asked for, it should output the date in some fallback format.

The width modifier, if present, indicates how many digits or letters should be included in the result. It takes the form
,m
or
,m-n
, where
m
is a number giving the minimum width, or
*
to indicate no minimum, and
n
is the maximum width, or
*
to indicate no maximum. If
n
is omitted,
*
is assumed.