How to calculate Previous Date from now date on c# ?

[calm calmy]

i have a program.
and i have some dates. they are:

1. 2011/08/01
2. 2011/08/02
3. 2011/08/03 (on this date there was an error on Database(for example),that can make user can't generate the report on that date)
4. 2011/08/05
5. 2011/08/06

and when it's on 2011/08/06 it's automatically generate the LatestSuccessDa te(2011/08/02) to (2011/08/05) on excel sheet. for information, I use HSSFWorkbook to generate it to excel.

this is a part code that i mean:

DateTime processDate = new DateTime(2011, 08, 06).AddDays(-1); 
DateTime LastSuccessdate = processDate.AddDays(1);

if (processDate > LastSuccessdate || processDate <= DateTime.Today.AddDays(-1))
{
   string destinationFolder = Settings.Default.CopyToDestinationFolder;
   string prefixFileName = Settings.Default.PrefixFileName;
   string[] excelColumnHeaderNames = Settings.Default.ExcelColumnHeaderNames.Split(',');

   IEnumerable<string[]> rows = loader.GetRows(sqlQuery, smsLastSucessDate, processDate);
   ExcelWriter excelWriter = new ExcelWriter(destinationFolder, prefixFileName);
   excelWriter.Write(excelColumnHeaderNames, rows, DateTime.Today.AddDays(-1));
}

[Christian Binder]

I don't know, what you want to do exactly, but you can subtract a day from now using DateTime dateBefore = DateTime.Now - new TimeSpan(1,0,0, 0);

[Plater]

Better still:
DateTime Yesterday=DateT ime.Now.AddDays (-1);