diff --git a/src/helper.cpp b/src/helper.cpp
index fbb4e4e6..7ebaba61 100644
--- a/src/helper.cpp
+++ b/src/helper.cpp
@@ -460,6 +460,8 @@ std::vector <Daterange> subtractRanges (
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+// From a set of intervals, find the earliest start and the latest end, and
+// return these in a Daterange.
 Daterange overallRangeFromIntervals (const std::vector <Interval>& intervals)
 {
   Daterange overall;