Why does formatting a DateTime as a string truncate and not round the milliseconds?

Question

When a Double is formatted as a string rounding is used. E.g.

Console.WriteLine(12345.6.ToString("F0"));

outputs

12346

However, when a DateTime is formatted as a string truncation is used. E.g.

var ci = CultureInfo.InvariantCulture;
var dateTime = DateTime.Parse("2011-09-14T15:18:42.999", ci);
Console.WriteLine(dateTime.ToString("o", ci));
Console.WriteLine(dateTime.ToString("s", ci));
Console.WriteLine(dateTime.ToString("yyyy-MM-hhThh:mm:ss.f", ci));

outputs

2011-09-14T15:18:42.9990000
2011-09-14T15:18:42
2011-09-14T15:18:42.9

What is the reasoning (if any) behind this behavior?

---

Rounding to nearest second can be achieved by adding half a second before formatting as a string:

var ci = CultureInfo.InvariantCulture;
var dateTime = DateTime.Parse("2010-12-31T23:59:59.999", ci);
Console.WriteLine(dateTime.ToString("s", ci));
var roundedDateTime = dateTime.AddMilliseconds(500);
Console.WriteLine(roundedDateTime.ToString("s", ci));

outputs

2010-12-31T23:59:59
2011-01-01T00:00:00

Answer 1

This is a bit subjective, but I would say that rounding date and times values as opposed to truncating them would result in a "more" unexpected behavior.

For example, rounding new DateTime(2011, 1, 1, 23, 59, 59, 999) would result in a new day completely. This sounds much more weird than just truncating the value.