How to round-off hours based on Minutes(hours+0 if min<30, hours+1 otherwise)?

Just as an alternative:

public static DateTime Round( DateTime dateTime )
{
    var updated = dateTime.AddMinutes( 30 );
    return new DateTime( updated.Year, updated.Month, updated.Day,
                         updated.Hour,  0, 0, dateTime.Kind );
}

If speed is an issue, the following should be the fastest way:

static DateTime RoundToHour(DateTime dt){
    long ticks = dt.Ticks + 18000000000;
    return new DateTime(ticks - ticks % 36000000000, dt.Kind);
}

It's also a pretty straight-forward and simple way to do it.

To explain, a DateTime structure doesn't actually have fields that store the year, month, day, hour, minute, etc. It stores one single long value, the number of "ticks" since a certain epoch (Jan 1, 1 AD). A tick is 100 nanoseconds, or one 10,000,000th of a second.

Any time you use any of the date/time properties, it divides by the proper constant.

So here, we add a constant equal to 30 minutes (30 * 60 * 1e7 = 18000000000 ticks), then subtract the remainder after dividing by a constant equal to one hour (60 * 60 * 1e7 = 36000000000 ticks).

What about:

public static DateTime RoundToHours(DateTime input)
{
DateTime dt = new DateTime(input.Year, input.Month, input.Day, input.Hour, 0, 0);

    if (input.Minute > 29)
      return dt.AddHours(1);
    else
      return dt;
}

No need to convert to string and back again!

EDIT:
Using a input.Hour+1 in the constructor will fail if the Hour is 23. The .AddHours(1) will correctly result in '0:00' the next day.