Is there a better way in C# to round a DateTime to the nearest 5 seconds?

Question

I want to round a DateTime to the nearest 5 seconds. This is the way I'm currently doing it but I was wondering if there was a better or more concise way?

DateTime now = DateTime.Now;
int second = 0;

// round to nearest 5 second mark
if (now.Second % 5 > 2.5)
{
    // round up
    second = now.Second + (5 - (now.Second % 5));
}
else
{
    // round down
    second = now.Second - (now.Second % 5);
}

DateTime rounded = new DateTime(now.Year, now.Month, now.Day, now.Hour, now.Minute, second);

Please note that I've found these two previous questions, however they truncate rather than round the time.

Solution 1

The Ticks count of a DateTime represents 100-nanosecond intervals, so you can round to the nearest 5 seconds by rounding to the nearest 50000000-tick interval like this:

  DateTime now = DateTime.Now;
  DateTime rounded = new DateTime(((now.Ticks + 25000000) / 50000000) * 50000000);

That's more concise, but not necessarily better. It depends on whether you prefer brevity and speed over code clarity. Yours is arguably easier to understand.

Solution 2

(Sorry for the resurrection; I recognize it's an old and answered question - just adding some extra code for Google's sake.)

I started with JayMcClellan's answer, but then I wanted it to be more generic, rounding to arbitrary intervals (not just 5 seconds). So I ended up leaving Jay's method for one that uses Math.Round on ticks and put it into an extension method that can take arbitrary intervals and also offers the option of changing the rounding logic (banker's rounding versus away-from-zero). I'm posting here in case this is helpful to someone else as well:

    public static TimeSpan Round(this TimeSpan time, TimeSpan roundingInterval, MidpointRounding roundingType) {
        return new TimeSpan(
            Convert.ToInt64(Math.Round(
                time.Ticks / (decimal)roundingInterval.Ticks,
                roundingType
            )) * roundingInterval.Ticks
        );
    }

    public static TimeSpan Round(this TimeSpan time, TimeSpan roundingInterval) {
        return Round(time, roundingInterval, MidpointRounding.ToEven);
    }

    public static DateTime Round(this DateTime datetime, TimeSpan roundingInterval) {
        return new DateTime((datetime - DateTime.MinValue).Round(roundingInterval).Ticks);
    }

It won't win any awards for bare efficiency, but I find it easy to read and intuitive to use. Example usage:

new DateTime(2010, 11, 4, 10, 28, 27).Round(TimeSpan.FromMinutes(1)); // rounds to 2010.11.04 10:28:00
new DateTime(2010, 11, 4, 13, 28, 27).Round(TimeSpan.FromDays(1)); // rounds to 2010.11.05 00:00
new TimeSpan(0, 2, 26).Round(TimeSpan.FromSeconds(5)); // rounds to 00:02:25
new TimeSpan(3, 34, 0).Round(TimeSpan.FromMinutes(37); // rounds to 03:42:00...for all your round-to-37-minute needs

Solution 3

Like you mentioned, it's fairly easy to truncate. So, just add 2.5 seconds, then truncate down.

Solution 4

How about this (blending a few answers together)? I think it conveys the meaning well and should handle the edge cases (rounding to the next minute) elegantly due to AddSeconds.

// truncate to multiple of 5
int second = 5 * (int) (now.Second / 5);
DateTime dt = new DateTime(..., second);

// round-up if necessary
if (now.Second % 5 > 2.5)
{
    dt = dt.AddSeconds(5);
}

The Ticks approach as shown by Jay is more concise, but may be a bit less readable. If you use that approach, at least reference TimeSpan.TicksPerSecond.

Solution 5

I can't think of a better way, although I would probably factor out the round method:

static int Round(int n, int r)
{
    if ((n % r) <= r / 2)
    {
        return n - (n % r); 
    }
    return n + (r - (n % r));
}

Also, % returns an int, so comparing it to 2.5 strikes me as a little odd, even though it is correct. I'd use >= 3.

View more solutions

Comments

[Damovisa]

If I add 2.5 seconds, truncate to the nearest 5 seconds and subtract the 2.5 seconds, I'll end up with 2.5sec, 7.5sec, 12.5sec etc...

[Matt Hamilton]

This works well because 59 seconds rounded to the nearest 5 will yield 60, which you can't pass as the 'seconds' parameter to the DateTime constructor. This way you avoid that pitfall.

[Damovisa]

Yeah, I know what you mean with the comparing it with 2.5 - it felt a little uncomfortable. As you say, it is correct though, and it makes it clearer what the intention is. 2.5 is clearly half of 5 while 3 seems not to fit.

[Ethan Callanan]

One potential pitfall, to criticize my own answer, is that I'm not sure how the DateTime accounts for leap-seconds. The tick count is measured from 12:00:00 midnight, January 1, 0001. So depending on the number of leap seconds since then and whether DateTime accounts for them, you might find that the resulting Seconds value is not a multiple of 5.

[Damovisa]

Wow, now that is detailed... I think I'm happy enough to ignore a potential error every approximately 18 months.

[Damovisa]

Yeah, fair point. I will be doing this - I just didn't make it clear in the question.

[Erich Mirabal]

you can make it more readable by using TimeSpan.TicksPerSecond.

[paxdiablo]

Or use 24999999 to get the behavior in the question, where multiples of 2.5 round down. Other than that, good solution. Is there any language that allows you to use commas in numbers to make them more readable? I would love to see this feature since numbers like 50000000 annoy the carp out of me. 50,000,000 would be so much better.

[Damovisa]

@Pax - you're right, the question will round down for anything between 2.5 and 3sec. It's not really how I need it to behave, I was just rushed.

[Joe]

-1: doesn't work if the original time contains fractions of a second.

[Erich Mirabal]

Thanks for keeping me honest :) I woke up this morning to this -1 and was like: "Argh! Stupid mistake. Don't think you can optimize while sleepy!"

[Erich Mirabal]

As a matter of curiosity, should the answers that result in an exception (from mishandling the case where second == 60) also not get voted down?

[Roman Starkov]

I prefer to round integers like this: ((n + (r>>1)) / r) * r (round midpoints up) or ((n + r>>1 - 1) / r) * r (round midpoints down). If I know r is odd I just use the first one because they work the same for odd r. This approach uses only one division (vs 3) and no branching compared to your function.

[porges]

JayMcClellan: It doesn't matter. .NET (and Windows) can't handle leap seconds.

[Stefan Steinegger]

You may improve readability by using TimeSpan.TicksPerSecond and Math.Round

[Gordon Thompson]

Bath time for you must be difficult ;-)

[paxdiablo]

Nice one, @Gordon, I know a fair bit more about C# nowadays but bathtime is pretty difficult. Though that's more to do with having young kids rather than any lack of coordination on my part :-)

[HappyNomad]

This is nice code for rounding to the nearest DateTime, but I also want the ability to round up to a multiple of roundingInterval. I tried modifying the DateTime overload to also take a MidpointRounding then pass it along to the other overload. But that didn't add the desired ability.

[newtothis]

@HappyNomad MidpointRounding only comes into play when the value is at the actual midpoint. If you always want to round up, you would need to add an overload or change the first function to use Math.Ceiling instead of Math.Round, and ignore the roundingType altogether.

[Nigel Touch]

Suggest your maintain the same Kind on the new DateTime you create.