The following characters are recognized in the format strings while creating / editing a Locale

a - "am" or "pm"
A - "AM" or "PM"
B - Swatch Internet time
c - ISO 8601 date (Requires PHP 5); i.e. "2004-02-12T15:19:21+00:00"
d - day of the month, 2 digits with leading zeros; i.e. "01" to "31"
D - day of the week, textual, 3 letters; i.e. "Fri"
e - day of the week, 1 letter; i.e. "F"
F - month, textual, long; i.e. "January"
g - hour, 12-hour format without leading zeros; i.e. "1" to "12"
G - hour, 24-hour format without leading zeros; i.e. "0" to "23"
h - hour, 12-hour format; i.e. "01" to "12"
H - hour, 24-hour format; i.e. "00" to "23"
i - minutes; i.e. "00" to "59"
I (capital i) - "1" if Daylight Savings Time, "0" otherwise.
j - day of the month without leading zeros; i.e. "1" to "31"
l (lowercase "L") - day of the week, textual, long; i.e. "Friday"
L - boolean for whether it is a leap year; i.e. "0" or "1"
m - month; i.e. "01" to "12"
M - month, textual, 3 letters; i.e. "Jan"
n - month without leading zeros; i.e. "1" to "12"
O - Difference to Greenwich time (GMT) in hours; i.e. "+0200"
r - RFC 822 formatted date; i.e. "Thu, 21 Dec 2000 16:01:07 +0200"
s - seconds; i.e. "00" to "59"
S - English ordinal suffix, textual, 2 characters; i.e. "th", "nd"
t - number of days in the given month; i.e. "28" to "31"
T - Timezone setting of this machine; i.e. "MDT"
U - seconds since the epoch
w - day of the week, numeric, i.e. "0" (Sunday) to "6" (Saturday)
W - ISO-8601 week number of year, weeks starting on Monday; i.e. "42"
Y - year, 4 digits; i.e. "1999"
y - year, 2 digits; i.e. "99"
z - day of the year; i.e. "0" to "365"
Z - timezone offset in seconds (i.e. "-43200" to "43200"). The offset for timezones west of UTC is always negative, and for those east of UTC is always positive.
isoZ - full ISO 8601 format, equivalent to Y-m-d\TH:i:s\Z
Unrecognized characters in the format string will be printed as-is.

You can escape characters by preceding them with a \ to print them as-is.