diff --git a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java
index d2abb36b0727..9f0bcd6ad82f 100644
--- a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java
+++ b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/CalendarInterval.java
@@ -34,6 +34,10 @@ public final class CalendarInterval implements Serializable, Comparable<Calendar
   public final int days;
   public final long microseconds;
 
+  // CalendarInterval is represented by months, days and microseconds. Months and days are not
+  // units of time with a constant length (unlike hours, seconds), so they are two separated fields
+  // from microseconds. One month may be equal to 29, 30 or 31 days and one day may be equal to
+  // 23, 24 or 25 hours (daylight saving)
   public CalendarInterval(int months, int days, long microseconds) {
     this.months = months;
     this.days = days;