16
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