locale.5: document era keywords

This patch completes the LC_TIME section - since these era
keywords are so tightly coupled, I'm providing them as a
single patch.

Based on
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap07.html
http://www.open-std.org/jtc1/SC22/WG20/docs/n972-14652ft.pdf

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/locale.5

@@ -1067,11 +1067,6 @@ The definition starts with the string
 in the first column.
 
 The following keywords are allowed:
-.\" FIXME The following LC_TIME keywords are not documented:
-.\"	era
-.\"	era_d_fmt
-.\"	era_d_t_fmt
-.\"	era_t_fmt
 .TP
 .I abday
 followed by a list of abbreviated names of the days of the week.
@@ -1125,6 +1120,89 @@ followed by the appropriate time format
 when using 12h clock format.
 This should be left empty for locales not using AM/PM convention.
 .TP
+.I era
+followed by semicolon-separated strings that define how years are
+counted and displayed for each era in the locale.
+Each string has the following format:
+.RS
+.PP
+.IR direction ":" offset ":" start_date ":" end_date ":" era_name ":" era_format
+.PP
+The fields are to be defined as follows:
+.PP
+.TP 4
+.I direction
+Either
+.BR +
+or
+.BR -.
+.BR +
+means the years closer to
+.IR start_date
+have lower numbers than years closer to
+.IR end_date .
+.BR -
+means the opposite.
+.TP
+.I offset
+The number of the year closest to
+.IR start_date
+in the era, corresponding to the
+.IR %Ey
+descriptor (see
+.BR strptime (3)).
+.TP
+.I start_date
+The start of the era in the form of
+.IR yyyy/mm/dd .
+Years prior AD 1 are represented as negative numbers.
+.TP
+.I end_date
+The end of the era in the form of
+.IR yyyy/mm/dd ,
+or one of the two special values of
+.BR -*
+or
+.BR +* .
+.BR -*
+means the ending date is the beginning of time.
+.BR +*
+means the ending date is the end of time.
+.TP
+.I era_name
+The name of the era corresponding to the
+.I %EC 
+descriptor (see
+.BR strptime (3)).
+.TP
+.I era_format
+The format of the year in the era corresponding to the
+.I %EY
+descriptor (see
+.BR strptime (3)).
+.RE
+.TP
+.I era_d_fmt
+followed by the format of the date in alternative era notation,
+corresponding to the
+.I %Ex
+descriptor (see
+.BR strptime (3)).
+.TP
+.I era_t_fmt
+followed by the format of the time in alternative era notation,
+corresponding to the
+.I %EX
+descriptor (see
+.BR strptime (3)).
+.TP
+.I era_d_t_fmt
+followed by the format of the date and time in alternative era notation,
+corresponding to the
+.I %Ec
+descriptor (see
+.BR strptime (3)).
+.TP
 .I alt_digits
 followed by the alternative digits used for date and time in the locale.
 .TP
@@ -1254,6 +1332,7 @@ This manual page isn't complete.
 .BR newlocale (3),
 .BR setlocale (3),
 .BR strftime (3),
+.BR strptime (3),
 .BR uselocale (3),
 .BR charmap (5),
 .BR charsets (7),