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));

Your examples don't round to the nearest. They truncate/round down. — CodesInChaos, Nov 10 '11 at 14:24
@CodeInChaos You are completely right. I will change the title because it's missleading. — Independent, Nov 10 '11 at 14:32
@VoidMain doesn't look right. You're setting the minutes to 0 with that, but keep seconds and fractions of a second. — CodesInChaos, Nov 10 '11 at 14:39
Yes, and it requires a descision inside the loop `Are there minutes, hours or day I have to clear, here` (Period.Type) — Independent, Nov 10 '11 at 14:53
You're righ @CodeInChaos but i was thinking among these lines, all the addXxxx in DateTime, return DateTime, you can chain it as much as you like and end up with something like this: d.AddHours( -d.Hour ).AddMinutes( -d.Minute ).AddSeconds( -d.Second ).AddMilliseconds( -d.Millisecond ); — VoidMain, Nov 11 '11 at 23:07

score 20 · Accepted Answer · edited Mar 05 '14 at 18:44

20

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)

edited Mar 05 '14 at 18:44

SetFreeByTruth

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answered Nov 10 '11 at 14:26

CodesInChaos

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Yes, very small performance difference between those good answers here. So I finally go to the most readable and clear way (because the functionality itself is a little obscure). I keep the foreach but through a "cleardate(date,period)"-method and immedietly after the data source. Which means the whole routine are already done by this loop. – Independent Nov 10 '11 at 15:06

score 19 · Answer 2 · answered Nov 10 '11 at 14:29

19

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)

answered Nov 10 '11 at 14:29

Svarog

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score 2 · Answer 3 · answered Aug 10 '15 at 07:35

I believe the following C# code will round a DateTime value to nearest minute, and I think it will be easy to generalize it to round to other units.

//round to nearest minute; add 30 seconds for rounding to nearest minute
effectiveDateTime = effectiveDateTime.AddSeconds(30);
TimeSpan timeComponent = effectiveDateTime.TimeOfDay;
effectiveDateTime = effectiveDateTime.Date;
effectiveDateTime = effectiveDateTime.AddHours(timeComponent.Hours).
     AddMinutes(timeComponent.Minutes);

score 2 · Answer 4 · answered Nov 10 '11 at 14:34

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);
    }
    // ...
}

score 1 · Answer 5 · answered Feb 20 '16 at 00:25

1

Not sure if this approach is effective, but looks quite nice using string format (in this case cutting down to hours):

var date = DateTime.UtcNow;
var cutDownDate = Convert.ToDateTime(date.ToString("yyyy-MM-dd hh"));

answered Feb 20 '16 at 00:25

Grengas

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score 0 · Answer 6 · edited May 23 '17 at 12:09

0

Answer Is there a better way in C# to round a DateTime to the nearest 5 seconds? contains an excellent generic DateTime rounding approach.

Edit: This answer was before the updated question title and is an algorithm for rounding to nearest not rounding down.

edited May 23 '17 at 12:09

Community

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answered Nov 10 '11 at 14:32

larsmoa

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Oh, I looked at that (and just the accepted answer). A nice feature from Matt W. It solve my question completely together with a little modification. RoundOff with TimeSpan.FromTicks(X). I get the ticks from Period.Month / Period.Days and whola, the call is generic! Though it does not provide any performance from my own sample that done double iteration. Developing are sometimes so erratic.. :) – Independent Nov 10 '11 at 14:49

score -1 · Answer 7 · answered Dec 22 '21 at 11:23

-1

Best method for day:

DateTime now = DateTime.Now;
DateTime roundDay = now.Date;

answered Dec 22 '21 at 11:23

NizMaS

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

7 Answers7