D-Link's rounding - Is this actually rouding?

Question

Some D-Link storage bays behave in an extremely odd way with (NTFS) timestamps.

Here is a quick sample from some experiments I did:

08:00:01.562 rounds to 08:00:01.000
08:00:01.633 rounds to 08:00:02.000
08:00:02.556 rounds to 08:00:03.000

I'm really puzzled by how 01.562 rounds down while 01.633 rounds up...

Can you understand what's going on here?

PS: Here's an extra table, if it helps.

Input           Output          Nearest second  FAT rounding
08:00:00.000    08:00:00.000    08:00:00.000    08:00:00.000
08:00:00.071    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.142    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.213    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.284    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.355    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.426    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.497    08:00:00.000    08:00:00.000    08:00:02.000
08:00:00.568    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.639    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.710    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.781    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.852    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.923    08:00:01.000    08:00:01.000    08:00:02.000
08:00:00.994    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.065    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.136    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.207    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.278    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.349    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.420    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.491    08:00:01.000    08:00:01.000    08:00:02.000
08:00:01.562    08:00:01.000    08:00:02.000    08:00:02.000
08:00:01.633    08:00:02.000    08:00:02.000    08:00:02.000
08:00:01.704    08:00:02.000    08:00:02.000    08:00:02.000
08:00:01.775    08:00:02.000    08:00:02.000    08:00:02.000
08:00:01.846    08:00:02.000    08:00:02.000    08:00:02.000
08:00:01.917    08:00:02.000    08:00:02.000    08:00:02.000
08:00:01.988    08:00:02.000    08:00:02.000    08:00:02.000
08:00:02.059    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.130    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.201    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.272    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.343    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.414    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.485    08:00:02.000    08:00:02.000    08:00:04.000
08:00:02.556    08:00:03.000    08:00:03.000    08:00:04.000
08:00:02.627    08:00:03.000    08:00:03.000    08:00:04.000
08:00:02.698    08:00:03.000    08:00:03.000    08:00:04.000
08:00:02.769    08:00:03.000    08:00:03.000    08:00:04.000

Answer 1

It may be just looking at the .5 and doing even/odd rounding. This helps keep things a bit sane for addition.

Lets say you have 1.5 and 2.5. If you add them directly, you get 1.5 + 2.5 => 4.0. But... if you round with normal ">= .5 goes up" then you'll get 2.0 + 3.0, and you get 5.0.

Lets change the rule a bit. If you're rounding n.m to integer, you can have a rule for .5 that includes n. For n.5, if n is even, you round up like normal. If n is odd, you round down. So for 1.5, you round down to 1. For 2.5, you round up (like normal) to 3. 1.0 + 3.0 = 4.0, like you'd expect.

Answer 2

This seems to be a bug, fixed in latest DLink NAS enclosures.