C# - Get Previous Day Using Datetime?

I want to set a DateTime property to previous day at 00:00:00. I don't know why DateTime.AddDays(-1) isn't working. Or why DateTime.AddTicks(-1) isn't working. First should this work?

I have 2 objects. Each object have DateTime fields ValidFrom, ValidTo.

EDIT: After coming home from work I tried to get the same behavior as my business objects behave. Below are the code I tried to replicate how it looks at work. Of course this is working at home but not at work. The good thing is I got good answers and +1 on all! =)

public class RuleValue
{
public DateTime ValidFrom, ValidTo;
public RuleValue(DateTime _validFrom, DateTime _validTo)
{

[Code]....