Source: A date in string format, for example, "2015-04-21".

The task: To find a date minus 1 day from the source.



string selDate = "2015-04-21"; 
DateTime selDateMinusOne = Convert.ToDateTime(selDate).AddDays(-1);

//Convert result date to the same format as source:
string strSelDateMinusOne = (Convert.ToDateTime(selDate).AddDays(-1)).ToString("yyyy-MM-dd"); 

The result is: 2015-04-20


SQL.  SQL date calculations:

In SQL the task like to calculate a date is very popular. For example, you need to get records between dates

The academic example: the query "select DATEADD(DD, -1, '2015-10-15')" will return '2015-10-14'

Practice example: select records since 1 day before '2015-10-15' until 1 day after this date:

OrderID, CustomerID, EmployeeID, OrderDate, RequiredDate
FROM [northwind].[dbo].[Orders]
  WHERE OrderDate BETWEEN  DATEADD(DD, -1, '2015-10-15') AND DATEADD(DD, +1, '2015-10-15')