Here is the standard way that I set default values for a DateTime field. I like to set defaults for my DateTime fields whenever it makes sense. In this example I am setting the values so that it will return all the data for the last month, including the current day.

public bool StandardMethodName(DateTime? startDate, DateTime? endDate)
{
     startDate = startDate.GetValueOrDefault(DateTime.Today.AddMonths(-1));
     endDate = endDate.GetValueOrDefault(DateTime.Now);

     return ProcessInfo(startDate, endDate);
}