C# date time helper for parsing "date + time + timezone offset in minutes" into the date that "I" want... ## the basic premise is that... - the user will pick a date using a date picker - the user will pick a time using a time picker - the moment js library will calculate the users timezone offset in minutes - based on their current browser sett…

DateTimeHelper.cs

public static class DateTimeHelper
{
    public static DateTimeOffset Combine(DateTimeOffset? originalDate, string timeString, int timeZoneOffsetInMinutes)
    {
        if(originalDate.HasValue)
        { 
            return FromString(GetDateAsString(originalDate.Value.Date, timeString, timeZoneOffsetInMinutes));
        }
        else
        {
            throw new InvalidOperationException("original must have a value");
        }
    }

    public static DateTime Combine(DateTime originalDate, string timeString, int timeZoneOffsetInMinutes)
    {
        return FromString(GetDateAsString(originalDate, timeString, timeZoneOffsetInMinutes)).DateTime;;
    }

    private static DateTimeOffset FromString(string dateToParse)  
    {
        return DateTimeOffset.ParseExact(dateToParse, "MM/dd/yyyy h:mm tt zzz", CultureInfo.InvariantCulture);
    }
    
    private static string GetDateAsString(DateTime dateComponent, string timeComponent, int offsetInMinutes)
    {
        var timeSpan = TimeSpan.FromMinutes(offsetInMinutes * -1);
        var offsetString = string.Format("{0:+00;-00}{1:00}", timeSpan.Hours, timeSpan.Minutes);
        return string.Format("{0} {1} {2}", dateComponent.ToString("MM/dd/yyyy"), timeComponent, offsetString);
    }
}

DateTimeHelperTest.cs

/// <summary>
/// the basic premise is that...
/// the user will pick a date using a date picker
/// the user will pick a time using a time picker
/// the moment js library will calculate the users timezone offset in minutes - based on their current browser setting
/// we need to take all this data and return a real DateTimeOffset or DateTime struct
/// this needs to work for standard, non-standard, and UTC time zones
/// </summary>
[TestFixture]
public class DateTimeHelperTest
{
    [TestCase("10:30 PM", "01/31/2013", 360, Description = "test MST time zone (UTC -6)")]
    [TestCase("4:30 PM", "01/31/2013", -120, Description = "tests E. Europe time zone (UTC +2)")]
    [TestCase("4:30 AM", "01/31/2013", -330, Description = "tests Mumbai time zone (UTC +5:30)")]
    [TestCase("04:30 PM", "01/31/2013", 0, Description = "tests Casablanca time zone (UTC)")] 
    public void Combine_Returns_Expected_DateTimeOffset(string timeString, string datePart, int offsetInMinutes)
    {
        var testDate = DateTimeOffset.Parse(datePart);

        var actual = DateTimeHelper.Combine(testDate, timeString, offsetInMinutes);

        Assert.AreEqual(GetExpected(timeString, datePart, offsetInMinutes), actual);
    }

    [TestCase("10:30 PM", "01/31/2013", 360, Description="test MST time zone (UTC -6)")]
    [TestCase("4:30 PM", "01/31/2013", -120, Description="tests E. Europe time zone (UTC +2)")]
    [TestCase("4:30 AM", "01/31/2013", -330, Description="tests Mumbai time zone (UTC +5:30)")]
    [TestCase("04:30 PM", "01/31/2013", 0, Description="tests Casablanca time zone (UTC)")] 
    public void CombineReturnsExpectedDate(string timeString, string datePart, int offsetInMinutes)
    {
        var testDate = DateTime.Parse(datePart);
            
        var actual = DateTimeHelper.Combine(testDate, timeString, offsetInMinutes);
            
        Assert.AreEqual(GetExpected(timeString, datePart, offsetInMinutes).DateTime, actual);
    }

    /// <summary>
    /// test method that mimics the code/algorithm under test
    /// </summary>
    private static DateTimeOffset GetExpected(string timeString, string datePart, int offsetInMinutes)
    {
        var timeSpan = TimeSpan.FromMinutes(offsetInMinutes * -1);
        var offsetString = string.Format("{0:+00;-00}{1:00}", timeSpan.Hours, timeSpan.Minutes);
        var expected = DateTimeOffset.ParseExact(string.Format("{0} {1} {2}", datePart, timeString, offsetString), "MM/dd/yyyy h:mm tt zzz", CultureInfo.InvariantCulture);
        return expected;
    }
}