Rounding DateTime objects

I want to round dates/times to the nearest interval for a charting application. I'd like an extension method signature like follows so that the rounding can be acheived for any level of accuracy:

static DateTime Round(this DateTime date, TimeSpan span);

The idea is that if I pass in a timespan of ten minutes, it will round to the nearest ten minute interval. I can't get my head around the implementation and am hoping one of you will have written or used something similar before.

I think either a floor, ceiling or nearest implementation is fine.

Any ideas?

Edit: Thanks to @tvanfosson & @ShuggyCoUk, the implementation looks like this:

public static class DateExtensions {
    public static DateTime Round(this DateTime date, TimeSpan span) {
        long ticks = (date.Ticks + (span.Ticks / 2) + 1)/ span.Ticks;
        return new DateTime(ticks * span.Ticks);
    }
    public static DateTime Floor(this DateTime date, TimeSpan span) {
        long ticks = (date.Ticks / span.Ticks);
        return new DateTime(ticks * span.Ticks);
    }
    public static DateTime Ceil(this DateTime date, TimeSpan span) {
        long ticks = (date.Ticks + span.Ticks - 1) / span.Ticks;
        return new DateTime(ticks * span.Ticks);
    }
}

And is called like so:

DateTime nearestHour = DateTime.Now.Round(new TimeSpan(1,0,0));
DateTime minuteCeiling = DateTime.Now.Ceil(new TimeSpan(0,1,0));
DateTime weekFloor = DateTime.Now.Floor(new TimeSpan(7,0,0,0));
...

Cheers!

Solution 1:

Floor

long ticks = date.Ticks / span.Ticks;

return new DateTime( ticks * span.Ticks, date.Kind );

Round (up on midpoint)

long ticks = (date.Ticks + (span.Ticks / 2) + 1)/ span.Ticks;

return new DateTime( ticks * span.Ticks, date.Kind );

Ceiling

long ticks = (date.Ticks + span.Ticks - 1)/ span.Ticks;

return new DateTime( ticks * span.Ticks, date.Kind );

Solution 2:

This will let you round to any interval given. It's also slightly faster than dividing and then multiplying the ticks.

public static class DateTimeExtensions
{
  public static DateTime Floor(this DateTime dateTime, TimeSpan interval)
  {
    return dateTime.AddTicks(-(dateTime.Ticks % interval.Ticks));
  }

  public static DateTime Ceiling(this DateTime dateTime, TimeSpan interval)
  {
    var overflow = dateTime.Ticks % interval.Ticks;

    return overflow == 0 ? dateTime : dateTime.AddTicks(interval.Ticks - overflow);
  }

  public static DateTime Round(this DateTime dateTime, TimeSpan interval)
  {
    var halfIntervalTicks = (interval.Ticks + 1) >> 1;

    return dateTime.AddTicks(halfIntervalTicks - ((dateTime.Ticks + halfIntervalTicks) % interval.Ticks));
  }
}