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

Question

I need to round-off the hours based on the minutes in a DateTime variable. The condition is: if minutes are less than 30, then minutes must be set to zero and no changes to hours, else if minutes >=30, then hours must be set to hours+1 and minutes are again set to zero. Seconds are ignored.

example:
11/08/2008 04:30:49 should become 11/08/2008 05:00:00
and 11/08/2008 04:29:49 should become 11/08/2008 04:00:00

I have written code which works perfectly fine, but just wanted to know a better method if could be written and also would appreciate alternative method(s).

string date1 = "11/08/2008 04:30:49";
DateTime startTime;
DateTime.TryParseExact(date1, "MM/dd/yyyy HH:mm:ss", null,     
    System.Globalization.DateTimeStyles.None, out startTime);

if (Convert.ToInt32((startTime.Minute.ToString())) > 29)
{
    startTime = DateTime.Parse(string.Format("{0}/{1}/{2} {3}:{4}:{5}",
        startTime.Month.ToString(), startTime.Day.ToString(), 
        startTime.Year.ToString(), startTime.Hour.ToString(), "00", "00"));
    startTime = startTime.Add(TimeSpan.Parse("01:00:00"));
    Console.WriteLine("startTime is :: {0}", 
        startTime.ToString("MM/dd/yyyy HH:mm:ss"));
}
else
{
    startTime = DateTime.Parse(string.Format("{0}/{1}/{2} {3}:{4}:{5}", 
        startTime.Month.ToString(), 
        startTime.Day.ToString(), startTime.Year.ToString(), 
        startTime.Hour.ToString(), "00", "00"));

        Console.WriteLine("startTime is :: {0}", 
        startTime.ToString("MM/dd/yyyy HH:mm:ss"));
}

Answer 1

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 );
}

Answer 2

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).