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

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);
    }
}