var dates = new[] { new DateTime(2004, 1, 1), new DateTime(2002, 1, 1), new DateTime(2003, 1, 1) };
var minDate = dates.Aggregate(dates.First(), (date, x) => new DateTime(Math.Min(date.Ticks, x.Ticks)));
// OR
var minDate2 = new DateTime(dates.Aggregate(dates.First().Ticks, (ticks, x) => Math.Min(ticks, x.Ticks)));
Console.WriteLine($"{minDate:u} {minDate2:u}");