diff --git a/content/html/content/src/HTMLInputElement.cpp b/content/html/content/src/HTMLInputElement.cpp
index 6e6d247769f..a4c4153e765 100644
--- a/content/html/content/src/HTMLInputElement.cpp
+++ b/content/html/content/src/HTMLInputElement.cpp
@@ -4671,10 +4671,18 @@ HTMLInputElement::GetValueAsDate(const nsAString& aValue,
     return false;
   }
 
-  uint32_t endOfYearOffset = aValue.Length() - 6;
+  uint32_t endOfYearOffset = 0;
+  for (; NS_IsAsciiDigit(aValue[endOfYearOffset]); ++endOfYearOffset);
 
-  if (aValue[endOfYearOffset]     != '-' || 
-      aValue[endOfYearOffset + 3] != '-') {
+  // The year must be at least 4 digits long.
+  if (aValue[endOfYearOffset] != '-' || endOfYearOffset < 4) {
+    return false;
+  }
+
+  // Now, we know where is the next '-' and what should be the size of the
+  // string.
+  if (aValue[endOfYearOffset + 3] != '-' ||
+      aValue.Length() != 10 + (endOfYearOffset - 4)) {
     return false;
   }