fix(datetime): clear out existing datetime data

Closes #6614

src/util/datetime-util.ts

@@ -261,6 +261,12 @@ export function updateDate(existingData: DateTimeData, newData: any) {
 
     // eww, invalid data
     console.warn(`Error parsing date: "${newData}". Please provide a valid ISO 8601 datetime format: https://www.w3.org/TR/NOTE-datetime`);
+
+  } else {
+    // blank data, clear everything out
+    for (var k in existingData) {
+      delete existingData[k];
+    }
   }
 }