Many students pay attention only to the numerical result of an operation and don't care anything more.

What is the difference ( 02/01/2000 - 28/12/1999) ?
2-28? 28-2? 31-28+2? ... (2;1;31;30;29) => 5

It is a number (time) not a date (another kind of "time").

Also computers (internal clock) convert dates in numbers (a long strings of bits) using a reference date. Sometimes, to avoid a lot of problems, it is very important to know what they (hardware and software) do.