var allIds = checkboxes.Select(x=>int.Parse(x.Split(new []{','})[0]));

if you want the oldest

int.Parse((checkboxes.OrderBy(x => DateTime.Parse(x.Split(new[] {','})[1])).First()).Split(new[] {','})[0]);

both cases will fail if checkboxes is empty. Please validate first and check the parsing of the date, you may have to pass a proper formatter