Extensions for GPS time

GpsTimeExtensions.cs

namespace System
{
    public static class GpsTime
    {
        public static int ToGpsTimeWeekNumber(this DateTime datetime)
        {
            DateTime datum = new DateTime(1980, 1, 6, 0, 0, 0);
            TimeSpan difference = datetime.Subtract(datum);
            return (int)(difference.TotalDays / 7);
        }

        public static int ToGpsTimeSeconds(this DateTime datetime)
        {
            DateTime datum = new DateTime(1980, 1, 6, 0, 0, 0);
            DateTime secondsdiff = datetime.AddDays(datetime.ToGpsTimeWeekNumber() * -7);
            TimeSpan difference = secondsdiff.Subtract(datum);
            return (int)difference.TotalSeconds;
        }

        public static DateTime FromGpsTime(int weeknumber, int seconds)
        {
            return new DateTime(1980, 1, 6, 0, 0, 0).AddDays(weeknumber * 7).AddSeconds(seconds);
        }
    }
}