diff --git a/README.md b/README.md
index f9775f6163c3df2137cf1f38e33805632ed10d1a..320357bdd92e96c7892126ef7bb7f862861c9032 100644
--- a/README.md
+++ b/README.md
@@ -184,7 +184,7 @@ AppleHealthKit.getStepCountForToday(null, (err: Object, steps: number) => {
 ___
 
 #### **`getStepCountForDay`**
-get the the aggregated total steps for the day provided as `date` in options object.
+get the the aggregated total steps for the day provided as `date` in options object. the `date` field expects an ISO date string as its value
 ```javascript
 let d = new Date(2016,5,27);
 let options = {