diff --git a/src/pure.js b/src/pure.js
index 6be6ccd..0085253 100644
--- a/src/pure.js
+++ b/src/pure.js
@@ -71,7 +71,7 @@ export function pruneHistory(entries, now = Date.now()) {
  *
  * @param {{ts:number, delta:number}[]} entries
  * @param {number} [now] Reference time in ms (defaults to Date.now()).
- * @returns {{trainsToday:number, sevenDayAvgPerDay:number, perDay:number}}
+ * @returns {{trainsToday:number, sevenDayAvgPerDay:number, perDay:number}} `perDay` is integer-rounded (floored); callers should not re-floor.
  */
 export function summary(entries, now = Date.now()) {
   if (entries.length === 0) {