diff --git a/reference/datetime/formats.xml b/reference/datetime/formats.xml
index fa77811b26..d3aa74bfb5 100644
--- a/reference/datetime/formats.xml
+++ b/reference/datetime/formats.xml
@@ -919,6 +919,14 @@ class DateTime#1 (3) {
     produces "2008-07-24 00:00". The reason for this is that those
     five statements directly influence the current time.
    </para>
+   <para>
+    Keywords such as "first day of" depend on the context in which the
+    relative format string is used. If used with a static method or function,
+    the referent is the current system timestamp. However, if used in
+    <function>DateTime::modify</function> or
+    <function>DateTimeImmutable::modify</function>, the referent is the object
+    on which the modify() method is called.
+   </para>
   </note>
   
   <note>