Update 'Data Design'

Data-Design.md

@@ -21,8 +21,8 @@ Design for the data in recklist
 
 - String: todo: used to hold the actual todo item
 - Boolean: done: is done
-- Date: due_by: due date
-- Date: completed_on: date it was checked off
+- u64: due_by: utc due date
+- u64: completed_on: utc date it was checked off
 - Vector: todos: sublist of todos
 
 ## Saving