diff --git a/doc/manual/source/the-model-file.rst b/doc/manual/source/the-model-file.rst
index 3685484cffb8763039ddd3b9c604c44e5abf5faa..afaf9b4c4d8255bb50d61e0992e91c134bf8d727 100644
--- a/doc/manual/source/the-model-file.rst
+++ b/doc/manual/source/the-model-file.rst
@@ -101,7 +101,10 @@ observed:
   quotes;
 * DATE indicates a time period which can be either a year (e.g. `2024Y` or
   `2024A`), a half-year (`2024S1` or `2024H1`), a quarter (`2024Q2`) or a month
-  (`2024M3`) (see :ref:`dates in a mod file`).
+  (`2024M3`) (see :ref:`dates in a mod file`). Optionally, the time period can
+  be followed by a plus sign and a number of periods, in which case the date is
+  shifted accordingly (e.g. `2023Q1+6` is accepted and is equivalent to
+  `2024Q3`).
 
 
 .. _var-decl: