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

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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.