Inconsistent DateTime to Unix Time conversion and error on 24 hour input

Question

Attached is a method I am currently using that takes in a list of DateTime strings, their input format (i.e. yyyy-MM-dd HH:mm:ss), and their offset in the form of hours.

As for the culture and "standard", I am using InvariantCulture and I am converting the times to UTC.

    public int unixFormat3(string dateTimeInput, string inputFormat, int hours)
    {
        DateTime result;
        CultureInfo provider = CultureInfo.InvariantCulture;
        result = DateTime.ParseExact(dateTimeInput, inputFormat, provider);

        int unixTime = (Int32)(result.ToUniversalTime().AddHours(hours).Subtract(new DateTime(1970, 1, 1, 0, 0, 0, 0, System.DateTimeKind.Utc))).TotalSeconds;
        return unixTime;
    }

Two issues with said method:

1. I am using this website as a comparison. If my input is 2014-03-18 21:00:00, my output, according to my method, is 1395190800, which converts back to 2014-03-19 01:00:00. It has a four hour difference. The desired output is this:

2. If my input is 2014-03-18 24:00:00, I get this error:

The DateTime represented by the string is not supported in calendar System.Globalization.GregorianCalendar.

Noticeably, it does not allow the input of 24 in the HH part. This is a weird error as NodaTime handles it just fine... Though that's irrelevant as I am using DateTime.

Does anyone have any insight on this area?

EDIT:

Upon some experimentation, removing the .ToUniversalTime() removes my 4-hour offset.. Why is this happening?

public int unixFormat3(string dateTimeInput, string inputFormat, int hours)
{
    DateTime result;
    CultureInfo provider = CultureInfo.InvariantCulture;
    result = DateTime.ParseExact(dateTimeInput, inputFormat, provider);

    int unixTime = (Int32)(result.AddHours(hours).Subtract(new DateTime(1970, 1, 1, 0, 0, 0, 0, System.DateTimeKind.Utc))).TotalSeconds;
    return unixTime;
}

Answer 1

This document, http://www.w3.org/TR/NOTE-datetime, cited in this question How to know whether a given string is a valid UTC DateTime format? does not list 24 as a valid hour value.

This document, http://en.wikipedia.org/wiki/Iso8601, cited by an answer to the question does list 24:00 as a valid time. This one, http://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noon_and_midnight, also says 24:00 is valid.

Answer 2

The System.DateTime object represents hours as an integer value between 0 and 23 (see http://msdn.microsoft.com/en-us/library/vstudio/system.datetime.hour(v=vs.100).aspx). As far as I know, NodaTime doesn't use any of the .NET provided DateTime or DateTimeOffset classes and handles everything itself, which is why it's handling an hour of 24 correctly.

As for why ToUniversalTime() is adding an offset, its probably because the ParseExact is returning a date that's already been adjusted. (What is the value of result just before you call ToUniversalTime()?)

You may also want to change your call to use this overload of ParseExact instead:

result = DateTime.ParseExact(dateTimeInput, inputFormat, provider, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal);

This tells the parser to assume the time is in UTC if no time zone is specified in the parsed string.

As a side note, you should probably declare your Unix epoch as a readonly global variable somewhere and use TryParseExact instead of ParseExact.

public class UnixTime
{
    public static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, 0, System.DateTimeKind.Utc);

    public int unixFormat3(string dateTimeInput, string inputFormat, int hours)
    {
        int unixTime = -1;
        DateTime result = DateTime.MinValue;
        if (DateTime.TryParseExact(dateTimeInput, inputFormat, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal, out result))
        {
            unixTime = (int)(result.AddHours(hours).Subtract(UnixTime.Epoch)).TotalSeconds;
        }

        return unixTime;
    }
}