diff --git a/src/date_func.h b/src/date_func.h
--- a/src/date_func.h
+++ b/src/date_func.h
@@ -24,6 +24,11 @@ void SetDate(Date date, DateFract fract)
 void ConvertDateToYMD(Date date, YearMonthDay *ymd);
 Date ConvertYMDToDate(Year year, Month month, Day day);
 
+/**
+ * Checks whether the given year is a leap year or not.
+ * @param yr The year to check.
+ * @return True if \c yr is a leap year, otherwise false.
+ */
 static inline bool IsLeapYear(Year yr)
 {
 	return yr % 4 == 0 && (yr % 100 != 0 || yr % 400 == 0);