[PATCH, v2] kbuild: doc: improve KBUILD_BUILD_TIMESTAMP documentation

[Gang Yan <gang.yan@linux.dev>]

From: Gang Yan <yangang@kylinos.cn>

This patch adds an example of how to set KBUILD_BUILD_TIMESTAMP to a
specific date. Also, note that the provided timestamp is used for
initramfs mtime fields, which are 32-bit and thus limited to dates
between the Unix epoch and 2106-02-07 06:28:15 UTC. Dates outside this
range will cause errors.

Suggested-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: David Disseldorp <ddiss@suse.de>
---
Changelog:
 v2:
  - Replace the invalid example with a valid one.
  - Apply David's suggestions.
---
 Documentation/kbuild/kbuild.rst | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index 3388a10f2dcc..881189ecd0ca 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -328,8 +328,14 @@ KBUILD_BUILD_TIMESTAMP
 ----------------------
 Setting this to a date string overrides the timestamp used in the
 UTS_VERSION definition (uname -v in the running kernel). The value has to
-be a string that can be passed to date -d. The default value
-is the output of the date command at one point during build.
+be a string that can be passed to date -d. E.g.::
+
+$ KBUILD_BUILD_TIMESTAMP="Mon Oct 13 00:00:00 UTC 2025" make
+
+The default value is the output of the date command at one point during
+build. If provided, this timestamp will also be used for mtime fields
+within any initramfs archive. Initramfs mtimes are 32-bit, so dates before
+the 1970 Unix epoch, or after 2106-02-07 06:28:15 UTC will fail.
 
 KBUILD_BUILD_USER, KBUILD_BUILD_HOST
 ------------------------------------
-- 
2.43.0