diff --git a/Documentation/kbuild/kbuild.txt b/Documentation/kbuild/kbuild.txt
index f11ebb3..646e2c1 100644
--- a/Documentation/kbuild/kbuild.txt
+++ b/Documentation/kbuild/kbuild.txt
@@ -205,7 +205,8 @@
 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 default value
+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.
 
 KBUILD_BUILD_USER, KBUILD_BUILD_HOST