ASP.NET/C#: Numerical Day of the Week

That gives me an error:
Error CS0103: The name 'Datetime' does not exist in the current context

I need something that will work with THIS same type of format:

Server.Time.ToString("%H")

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*No Points*

DateTime is in the System namespace, so be sure to bring that in somehow.

e.g.

System.Datetime.Now.DayofWeek + 1

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Use the +1 if you want to start at one. The DayOfWeek enum starts its values at zero (i.e. Sunday = 0).

How can I do this without using the system interface?  
                   
For example, sometimes I need to access past dates, not just NOW, like this:
Print(PingPrevious.ClosingTime.ToString("%H"));

I am looking for a way to use this with .ToString(()