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Dealing with hours with no minutes when rounding DateTime to the 30 minutes
protected DateTime GetStartTime()
{
    var spanTicks = TimeSpan.FromMinutes(30).Ticks;
    var ticks = (Time.Now.Ticks + spanTicks - 1) / spanTicks;

    return new DateTime(ticks * spanTicks);
}

var now = DateTime.Now;
DateTime result = now.AddMinutes(now.Minute >= 30 ? (60-now.Minute) : (30-now.Minute));
result = result.AddSeconds(-1* result.Second); // To reset seconds to 0
result = result.AddMilliseconds(-1* result.Millisecond); // To reset milliseconds to 0

protected DateTime GetStartTime() {
    var now = DateTime.Now;
    int addHours;
    int minute;
    if (now.Minute >= 30) {
        addHour = 1;
        minute = 0;
    } else {
        addHour = 0;
        minute = 30;
    }
    return new DateTime(now.Year, now.Month, now.Day, hour, minute, 0).AddHours(addHours);
}

protected DateTime GetStartTime()
{
    DateTime dt=DateTime.Now;
    if (dt.Minute<30)
    {
        dt=dt.AddMinutes(30-dt.Minute);
    }
    else
    {
        dt=dt.AddMinutes(60-dt.Minute);
    }

    //dt now has the upcoming half-hour border

    //...

}

private static readonly long _ticksIn30Mins = TimeSpan.FromMinutes(30).Ticks;

protected DateTime GetRoundedTime(DateTime inputTime)
{
    long currentTicks = inputTime.Ticks;
    return new DateTime(currentTicks.RoundUp(_ticksIn30Mins));
}

public static class ExtensionMethods
{
    public static long RoundUp(this long i, long toTicks)
    {
        return (long)(Math.Round(i / (double)toTicks,
                  MidpointRounding.AwayFromZero)) * toTicks;
    }
}

var currentTime = DateTime.Now;
var rounded = GetRoundedTime(currentTime);
if (rounded == currentTime)
{
    rounded = new DateTime(rounded.Ticks + _ticksIn30Mins);
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        private static readonly long _ticksIn30Mins = TimeSpan.FromMinutes(30).Ticks;

        static void Main(string[] args)
        {
            WriteDateString(new DateTime(2012, 01, 18, 09, 45, 11, 152));
            WriteDateString(new DateTime(2012, 01, 18, 12, 15, 11, 999));
            WriteDateString(new DateTime(2012, 01, 18, 12, 00, 00, 000));

            Console.ReadLine();
        }

        private static void WriteDateString(DateTime dateTime)
        {
            Console.WriteLine("Before: {0}, After: {1}", dateTime, GetRoundedTime(dateTime));
        }

        private static DateTime GetRoundedTime(DateTime inputTime)
        {
            long currentTicks = inputTime.Ticks;
            var rounded = new DateTime(currentTicks.RoundUp(_ticksIn30Mins));
            if (rounded == inputTime)
            {
                rounded = new DateTime(rounded.Ticks + _ticksIn30Mins);
            }
            return rounded;
        }
    }

    public static class ExtensionMethods
    {
        public static long RoundUp(this long i, long toTicks)
        {
            return (long)(Math.Round(i / (double)toTicks, MidpointRounding.AwayFromZero)) * toTicks;
        }
    }
}