Is there a simple function for rounding a DateTime down to the nearest 30 minutes, in C#?

Question

For example:

2011-08-11 16:59 becomes 2011-08-11 16:30

Answer 1

Here's how I handled it. Below is the base method and an overload to round to the nearest 30 minutes:

public static DateTime RoundMinutes(this DateTime value)
{
    return RoundMinutes(value, 30);
}

public static DateTime RoundMinutes(this DateTime value, int roundMinutes)
{
    DateTime result = new DateTime(value.Ticks);

    int minutes = value.Minute;
    int hour = value.Hour;
    int minuteMod = minutes % roundMinutes;

    if (minuteMod <= (roundMinutes / 2))
    {
        result = result.AddMinutes(-minuteMod);
    }
    else
    {
        result = result.AddMinutes(roundMinutes - minuteMod);
    }

    return result;
}

Answer 2

Exploiting some math

var input = DateTime.Now; // or however you get DateTime
var rounded = input.AddMinutes(-input.TimeOfDay.TotalMinutes % 30d);

Note that TimeOfDay is a TimeSpan and its TotalMinutes property is a double and that the modulus operator functions on doubles like follows:

47.51 % 30d == 17.51

16.2 % 30d == 16.2

768.7 % 30d == 18.7

You could change the 30d to any value you like other than zero. Changing to 15 rounds down to 15 minute intervals for instance. Changing from 30d to -30d, didn't change the results from the tests that I ran.

You could create a rounding extension method (providing this rounding method for all DateTime values):

public static class DateTimeExtensions
{
    public static DateTime Round(this DateTime self, double minutesInInterval)
    {
        if (minutesInInterval == 0d) throw new ArgumentOutOfRangeException("minutesInInterval");
        return self.AddMinutes(-self.TimeOfDay.TotalMinutes % minutesInInterval);
    }
}

Answer 3

DateTime RoundDown(DateTime dt, TimeSpan d)
{
    return new DateTime((dt.Ticks / d.Ticks) * d.Ticks);
}

Example:

var dt1 = RoundDown(DateTime.Parse("2011-08-11 16:59"), TimeSpan.FromMinutes(30));
// dt1 == {11/08/2011 16:30:00}

var dt2 = RoundDown(DateTime.Parse("2011-08-11 17:00"), TimeSpan.FromMinutes(30));
// dt2 == {11/08/2011 17:00:00}

var dt3 = RoundDown(DateTime.Parse("2011-08-11 17:01"), TimeSpan.FromMinutes(30));
// dt3 == {11/08/2011 17:00:00}

Answer 4

A more generic solution rounding to the nearest time span:

public static DateTime RoundUp(this DateTime dt, TimeSpan d)
{
    var delta = (d.Ticks - (dt.Ticks % d.Ticks)) % d.Ticks;
    return new DateTime(dt.Ticks + delta);
}

public static DateTime RoundDown(this DateTime dt, TimeSpan d)
{
    var delta = dt.Ticks % d.Ticks;
    return new DateTime(dt.Ticks - delta);
}

public static DateTime RoundToNearest(this DateTime dt, TimeSpan d)
{
    var delta = dt.Ticks % d.Ticks;
    bool roundUp = delta > d.Ticks / 2;

    return roundUp ? dt.RoundUp(d) : dt.RoundDown(d);
}

It would be used this way:

var date = new DateTime(2010, 02, 05, 10, 35, 25, 450); // 2010/02/05 10:35:25
var rounded = date.RoundToNearest(TimeSpan.FromMinutes(30)); // 2010/02/05 10:30:00

More in this response.

Answer 5

DateTime newDateTime = new DateTime(oldDateTime.Year, oldDateTime.Month, oldDateTime.Day
  ,oldDateTime.Hour, (oldDateTime.Minute / 30) * 30, 0);

Answer 6

I would say something like that

var time = DateTime.Now;
var rounded = time.AddMinutes(
        time.Minute>30 ? -(time.Minute-30) : -time.Minute) 

And you could even do your own extension

public static class TimeHelper {
   public static DateTime RoundDown (this DateTime time)
   {
       return time.AddMinutes(
         time.Minute>30 ? -(time.Minute-30) : -time.Minute);
   }
}

EDIT

This function cut's also the seconds / milliseconds if necessary. Thanks for the hint.

public static DateTime RoundDown(this DateTime time)
{
    return time.Subtract(
        new TimeSpan(0, 0, time.Minute > 30 ? (time.Minute - 30) : time.Minute, 
            time.Second, time.Millisecond));
}