Question

For example:

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

Answer 1

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));
}