format-time-string: document new '+' flag

* doc/lispref/os.texi (Time Parsing), etc/NEWS:
* src/timefns.c (Fformat_time_string): Document the new
behavior, added for compatibility with POSIX.1-2017.

1 files changed, 6 insertions, 2 deletions

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index cb8f25df0a3..59cd5a8fe8a 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1600,7 +1600,9 @@ This is a synonym for @samp{%m/%d/%y}.
 @item %e
 This stands for the day of month, blank-padded.
 @item %F
-This stands for the ISO 8601 date format, i.e., @samp{"%Y-%m-%d"}.
+This stands for the ISO 8601 date format, which is like
+@samp{%+4Y-%m-%d} except that any flags or field width override the
+@samp{+} and (after subtracting 6) the @samp{4}.
 @item %g
 This stands for the year corresponding to the ISO week within the century.
 @item %G
@@ -1680,7 +1682,9 @@ This stands for a single @samp{%}.
 @end table
 
 One or more flag characters can appear immediately after the @samp{%}.
-@samp{0} pads with zeros, @samp{_} pads with blanks, @samp{-}
+@samp{0} pads with zeros, @samp{+} pads with zeros and also puts
+@samp{+} before nonnegative year numbers with more than four digits,
+@samp{_} pads with blanks, @samp{-}
 suppresses padding, @samp{^} upper-cases letters, and @samp{#}
 reverses the case of letters.