Converter

using System;
using System.Globalization;
using DatafoxTSLibrary.Terminals;
using System.Windows.Data;

namespace AdminClientv03.Converters
{
	class DtTermToTimeDiffernce : IValueConverter
	{
		public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
		{
			var term = value as DatafoxTerminal;
			if (value == null)
				return 0;

			if (term.AskedAt.HasValue && term.CurrentTimeOnTerminal.HasValue)
				return (term.AskedAt.Value - term.CurrentTimeOnTerminal.Value).TotalSeconds;
			else
				return 0;
		}

		public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
		{
			throw new NotImplementedException("Only one way conversion supported");
		}
	}
}

XAML

<conv:DtTermToTimeDiffernce x:Key="DtConv"/>