Это действительно вопрос того, чей 2010-Jan-01 вы используете и хотите ли вы учесть переход на летнее время.
//I'm currently in Central Daylight Time (Houston, Texas)
DateTime jan1 = new DateTime(2010, 1, 1);
//days since Jan1 + time since midnight
TimeSpan differenceWithDaylightSavings = DateTime.Now - jan1;
//one hour less than above (we "skipped" those 60 minutes about a month ago)
TimeSpan differenceWithoutDaylightSavings = (DateTime.UtcNow - jan1.ToUniversalTime());
//difference for those using UTC and 2010-Jan-01 12:00:00 AM UTC as their starting point
// (today it's 5 hours longer than differenceWithDaylightSavings)
TimeSpan utcDifference = (DateTime.UtcNow - new DateTime(2010, 1, 1));
Difference with Daylight Savings: 105.15:44:09.7003571
Difference without Daylight Savings: 105.14:44:09.7003571
UTC Difference: 105.20:44:09.7003571
Чтобы получить секунды, используйте свойство TotalSeconds для объекта TimeSpan.