Answer: total order
Step-by-step explanation: A total order set is a set and it's relation which satisfies the conditions for a partial order and the comparability relation. What this means is two number in general have two conditions either one is less than or equal to, it doesn't matter which two numbers we select. So a total order set is just like ≤ for numbers. Since X birth date is earlier than Y birth date this satisfies the conditions for total order X is ≤ Y (less than or equal Y).