diff --git a/bin/dnssec/dnssec-signzone.rst b/bin/dnssec/dnssec-signzone.rst
index 3e1465a43c..fffae1980a 100644
--- a/bin/dnssec/dnssec-signzone.rst
+++ b/bin/dnssec/dnssec-signzone.rst
@@ -174,6 +174,11 @@ Options
    days. Therefore, if any existing RRSIG records are due to expire in
    less than 7.5 days, they are replaced.
 
+   Note that the calculation of cycle interval is based upon the validity
+   period of the replacement signatures that would be generated by
+   ``dnssec-signzone``, not on the valid lifetimes of the input RRSIGs being
+   considered for pre-expiry replacement.
+
 .. option:: -I input-format
 
    This option sets the format of the input zone file. Possible formats are