Cut-/Rounding down a DateTime to a Whole i.e. Hour/Day?

Question

Is there any kind of mathematical way to cut DateTime down to a exact Hour, Day or so? Similiar to round of a decimal to int.

Period.Day
If the original value was 2011-01-01 13:00:00, it ends up in 2011-01-01 00:00:00

if Period.Hour
If the original value was 2011-03-11 13:32:00, it ends up in 2011-03-11 13:00:00

I think about something like below. This are of course works fine, but the range-array are iterated through anyway, later. Better if I was possible to calculate directly on that iteration, instead of it's own. But someType can't be put into that iteration (it depends on someType).

if (someType == Period.Day)
  range.ForEach(d => d.time = new DateTime(d.time.Year, d.time.Month, d.time.Day,0,0,0));
if (someType == Period.Hour)
  range.ForEach(d => d.time = new DateTime(d.time.Year, d.time.Month, d.time.Day, d.time.Hour, 0, 0));

Answer 1

Rounding down to a day is equivalent to time.Date, rounding to nearest (up on midpoint) is simply ( time + 12hours ).Date.

For rounding down to a full hour I can't think of code that's nicer to read than yours. For rounding up to the nearest hour you can apply your code to time + 30mins.

There is probably a faster method for rounding to the nearest hour:

const Int64 HourInTicks=...;
Int64 timeInTicks=time.Ticks;
Int64 trucatedToHour=timeInTicks-timeInTicks%HourInTicks;

But I'd avoid that, unless you really need the performance, which is unlikely.

(My round to nearest might have issues on days where the local time offset changes if you're using local time)

Answer 2

To round down to day you can use the DateTime.Date Property.
To round down to hour, I'm afraid you'll have to either use what you did in your example or something like:

d.Date.AddHours(d.Hour)

Answer 3

I'll do the following:

private static readonly DateTime Epoch = new DateTime(1970, 1, 1); 
public static DateTime Round(this DateTime d, Period p)
{
    var ts = d - Epoch;

    if (p == Period.Hour)
    {
        var hours = (long)ts.TotalHours;
        return Epoch.AddHours(hours);
    }
    else if (p == Period.Days)
    {
        var days = (long)ts.TotalDays;
        return Epoch.AddDays(days);
    }
    // ...
}