date: attempt to more accurately describe year limitations with -v

The previous description was both incorrect and incomplete in its
description -- the 2038 limit doesn't apply on !i386 platforms, and
it didn't note that values above 100 are accepted and interpreted
differently.  Further, it didn't note that absolute years are accepted.

Reviewed by:	pauamma_gundo.com (manpages)
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D35360

1 files changed, 10 insertions, 2 deletions

diff --git a/bin/date/date.1 b/bin/date/date.1
index 7254e7e83e3b..da2dadd13c00 100644
--- a/bin/date/date.1
+++ b/bin/date/date.1
@@ -32,7 +32,7 @@
 .\"     @(#)date.1	8.3 (Berkeley) 4/28/95
 .\" $FreeBSD$
 .\"
-.Dd November 3, 2021
+.Dd May 31, 2022
 .Dt DATE 1
 .Os
 .Sh NAME
@@ -227,7 +227,15 @@ seconds are in the range 0-59, minutes are in the range 0-59, hours are
 in the range 0-23, month days are in the range 1-31, week days are in the
 range 0-6 (Sun-Sat),
 months are in the range 1-12 (Jan-Dec)
-and years are in the range 80-38 or 1980-2038.
+and years are in a limited range depending on the platform.
+.Pp
+On i386, years are in the range 69-38 representing 1969-2038.
+On every other platform, years 0-68 are accepted and represent 2000-2068, and
+69-99 are accepted and represent 1969-1999.
+In both cases, years between 100 and 1900 (both included) are accepted and
+interpreted as relative to 1900 of the Gregorian calendar with a limit of 138 on
+i386 and a much higher limit on every other platform.
+Years starting at 1901 are also accepted, and are interpreted as absolute years.
 .Pp
 If
 .Ar val