diff --git a/README.md b/README.md
index 77991cffff4e3217fff991f441881203d6bd2bce..815fd7477b756cb82cd0d59864f1b287f87bc8e2 100644
--- a/README.md
+++ b/README.md
@@ -49,7 +49,7 @@ calculate_statistics(
 
     :param sampling: temporal aggregation, one of ``daily``,
                      ``monthly``, ``seasonal``, ``vegseason``,
-                     ``summer``, ``xsummer``, or ``annual``;
+                     ``summer``, ``xsummer``, ``annual``, or ``custom``;
                      ``summer`` will pick the 6-months summer season in
                      the hemisphere where the station is located;
                      ``xsummer`` does the same for a 7-months summer
@@ -57,7 +57,9 @@ calculate_statistics(
                      ``vegseason`` requires also the ``crops`` argument
                      and will then determine the appropriate growing
                      seasons based on the ``climatic_zone`` metadata and
-                     crop type
+                     crop type;
+                     ``custom`` will create one aggregate value over the
+                     entire time series
     :param statistics: a single statistic or metric or a list of
                        statistics and metrics to call, these must be
                        defined in ``stats.py`` or ``ozone_metrics.py``
diff --git a/toarstats/metrics/interface.py b/toarstats/metrics/interface.py
index 28c88ae8c6bc14e6f2adfec793fb7dfe43844445..8bbccf6973a7b85643e15525d027f2a9d026d347 100644
--- a/toarstats/metrics/interface.py
+++ b/toarstats/metrics/interface.py
@@ -34,7 +34,9 @@ def calculate_statistics(
                      ``vegseason`` requires also the ``crops`` argument
                      and will then determine the appropriate growing
                      seasons based on the ``climatic_zone`` metadata and
-                     crop type
+                     crop type;
+                     ``custom`` will create one aggregate value over the
+                     entire time series
     :param statistics: a single statistic or metric or a list of
                        statistics and metrics to call, these must be
                        defined in ``stats.py`` or ``ozone_metrics.py``